Method and device for transmitting reference signal in multi-antenna supporting wireless communication system

ABSTRACT

The present invention relates to a method and a device for estimating a channel of a terminal in a wireless communication system that supports multiple antennas. Particularly, the method comprises the steps of receiving a channel state information-reference signal (CSI-RS) for a plurality of first domain antennas and a cell-specific reference signal (CRS) for a plurality of second domain antennas, and estimating the all of the channels on the basis of a first channel for the first domain antennas estimated from the CSI-RS and a second channel for the second domain antennas estimated from the CRS.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method and device for transmitting a referencesignal in a multi-antenna supporting wireless communication system.

BACKGROUND ART

MIMO (multiple-input multiple-output) technology means a method ofimproving data transfer efficiency by adopting multiple transmittingantennas and multiple receiving antennas instead of using a singletransmitting antenna and a single receiving antenna. In particular, theMIMO technology is the technology for improving capacity or performanceof a transmitting or receiving end of a wireless communication system byusing multiple antennas. In addition, the MIMO technology can bereferred to as a multi-antenna technology.

To support MIMO transmission, a precoding matrix can be applied toproperly distribute transmission information to each antenna accordingto a channel status and the like. In the conventional 3GPP (3^(rd)generation partnership project) LTE (long term evolution) system,maximum 4 transmit antennas (4Tx) are supported for downlinktransmission and a related precoding codebook is defined.

DISCLOSURE OF THE INVENTION Technical Task

Based on the above-described discussion, a method and device fortransmitting a reference signal in a wireless communication systemsupporting multiple antennas are proposed.

Technical tasks obtainable from the present invention are non-limited bythe above-mentioned technical task. And, other unmentioned technicaltasks can be clearly understood from the following description by thosehaving ordinary skill in the technical field to which the presentinvention pertains.

Technical Solutions

In a first technical aspect of the present invention, provided herein isa method of estimating a channel by a user equipment in a wirelesscommunication system supporting multiple antennas, including: receivinga CSI-RS (channel state information-reference signal) for a multitude offirst domain antennas and a CRS (cell-specific reference signal) for amultitude of second domain antennas and; estimating entire channelsbased on both a first channel for the first domain antennas estimatedfrom the CSI-RS and a second channel for the second domain antennasestimated from the CRS.

Preferably, the entire channels may be determined by Kronecker productof the first channel and the second channel.

Preferably, when the first domain antennas are vertical domain antennas,the second domain antennas may be horizontal domain antennas. On thecontrary, when the first domain antennas are the horizontal domainantennas, the second domain antennas may be the vertical domainantennas.

Preferably, the CSI-RS and the CRS may be configured through RRC (radioresource control) signaling.

Preferably, the CRS may be a CRS quasi co-located with the CSI-RS or aCRS of a serving cell.

Preferably, the method may further include feeding back channel stateinformation on the entire channels to a base station.

In a second technical aspect of the present invention, provided hereinis a method of estimating a channel by a user equipment in a wirelesscommunication system supporting multiple antennas, including: receivinga CSI-RS (channel state information-reference signal) for a multitude ofvertical antennas and first horizontal antennas and a CRS (cell-specificreference signal) for second horizontal antennas; and estimating entirechannels based on both a vertical antenna channel estimated from theCSI-RS and a horizontal antenna channel estimated from the CSI-RS andthe CRS.

Preferably, the entire channels may be determined by Kronecker productof the vertical antenna channel and the horizontal antenna channel.

In a third technical aspect of the present invention, provided herein isa user equipment for performing channel estimation in a wirelesscommunication system supporting multiple antennas, including: a radiofrequency unit and a processor. The processor may be configured toreceive a CSI-RS (channel state information-reference signal) for amultitude of first domain antennas and a CRS (cell-specific referencesignal) for a multitude of second domain antennas and estimate entirechannels based on both a first channel for the first domain antennasestimated from the CSI-RS and a second channel for the second domainantennas estimated from the CRS.

Advantageous Effects

According to embodiments of the present invention, a reference signalcan be efficiently transmitted in a wireless communication systemsupporting multiple antennas.

It will be appreciated by persons skilled in the art that that theeffects achieved by the present invention are not limited to what hasbeen particularly described hereinabove and other advantages of thepresent invention will be more clearly understood from the followingdetailed description.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention.

FIG. 1 is a diagram to describe a structure of a downlink radio frame.

FIG. 2 is a diagram for one example of a resource grid for one downlinkslot.

FIG. 3 is a diagram for a structure of a downlink subframe.

FIG. 4 is a diagram for a structure of an uplink subframe.

FIG. 5 is a diagram for a pattern of a common reference signal (CRS).

FIG. 6 is a diagram to describe a shift of a reference signal pattern.

FIG. 7 and FIG. 8 are diagrams to describe a resource element group(REG) corresponding to a unit to which downlink control channels areassigned.

FIG. 9 is a diagram to illustrate a scheme of transmitting PCFICH.

FIG. 10 is a diagram for locations of PCFICH and PHICH.

FIG. 11 is a diagram for a location of a downlink resource element towhich a PHICH group is mapped.

FIG. 12 is a diagram for a structure of a transmitter according to anSC-FDMA scheme

FIG. 13 is a diagram to describe a scheme of mapping a DFT-processedsignal into a frequency domain.

FIG. 14 is a block diagram to describe a process for reference signaltransmission.

FIG. 15 is a diagram for a location of a symbol to which a referencesignal is mapped.

FIGS. 16 to 19 are diagrams to describe a clustered DFT-s-OFDMA scheme.

FIG. 20 is a diagram for a structure of an MIMO system.

FIG. 21 is a block diagram to describe functionality of an MIMO system.

FIG. 22 is a diagram to describe a basic concept of codebook basedprecoding.

FIG. 23 is a diagram for examples of configuring 8 transmittingantennas.

FIG. 24 is a reference diagram of a 2-dimensional active antenna systemaccording to the present invention.

FIG. 25 is a reference diagram for explaining an embodiment of thepresent invention.

FIGS. 26 and 27 illustrate embodiments for feeding back all channelsbased on CRS and CSI-RS according to the present invention.

FIG. 28 illustrates an embodiment for using CSI-RS and CRSsimultaneously for specific domain antennas according to the presentinvention.

FIG. 29 is a block diagram illustrating configurations of a base stationdevice and a user equipment device according to the present invention.

BEST MODE FOR INVENTION

The following embodiments are achieved by combination of structuralelements and features of the present invention in a predetermined type.Each of the structural elements or features should be consideredselectively unless specified separately. Each of the structural elementsor features may be carried out without being combined with otherstructural elements or features. Also, some structural elements and/orfeatures may be combined with one another to constitute the embodimentsof the present invention. The order of operations described in theembodiments of the present invention may be changed. Some structuralelements or features of one embodiment may be included in anotherembodiment, or may be replaced with corresponding structural elements orfeatures of another embodiment.

In this specification, the embodiments of the present invention havebeen described based on the data transmission and reception between abase station BS and a user equipment UE. In this case, the base stationBS means a terminal node of a network, which performs directcommunication with the user equipment UE. A specific operation which hasbeen described as being performed by the base station may be performedby an upper node of the base station BS as the case may be.

In other words, it will be obvious to those skilled in the art thatvarious operations for enabling the base station to communicate with theterminal in a network composed of several network nodes including thebase station will be conducted by the base station or other networknodes other than the base station. The term “Base Station (BS)” may bereplaced with a fixed station, Node-B, eNode-B (eNB), or an access point(AP) as necessary. The term “relay” may be replaced with a Relay Node(RN) or a Relay Station (RS). The term “terminal” may also be replacedwith a User Equipment (UE), a Mobile Station (MS), a Mobile SubscriberStation (MSS) or a Subscriber Station (SS) as necessary. In the presentspecification, an uplink transmitter may be a UE or a relay and anuplink receiver may be a BS or a relay. Similarly, a downlinktransmitter may be a BS or a relay and a downlink receiver may be a UEor a relay. In other words, uplink transmission may mean transmissionfrom a UE to a BS, transmission from a UE to a relay, or transmissionfrom a relay to a BS. Similarly, downlink transmission may meantransmission from a BS to a UE, transmission from a BS to a relay ortransmission from a relay to a UE.

Specific terminologies hereinafter used in the embodiments of thepresent invention are provided to assist understanding of the presentinvention, and various modifications may be made in the specificterminologies within the range that they do not depart from technicalspirits of the present invention.

In some cases, to prevent the concept of the present invention frombeing ambiguous, structures and apparatuses of the known art will beomitted, or will be shown in the form of a block diagram based on mainfunctions of each structure and apparatus. Also, wherever possible, thesame reference numbers will be used throughout the drawings and thespecification to refer to the same or like parts.

The embodiments of the present invention may be supported by standarddocuments disclosed in at least one of wireless access systems, i.e.,IEEE 802 system, 3GPP system, 3GPP LTE system, 3GPP LTE, 3GPP LTE-A(LTE-Advanced) system, and 3GPP2 system. Namely, among the embodimentsof the present invention, apparent steps or parts, which are notdescribed to clarify technical spirits of the present invention, may besupported by the above documents. Also, all terminologies disclosedherein may be described by the above standard documents.

The following technologies can be applied to a variety of wirelessaccess technologies, for example, CDMA (Code Division Multiple Access),FDMA (Frequency Division Multiple Access), TDMA (Time Division MultipleAccess), OFDMA (Orthogonal Frequency Division Multiple Access), SC-FDMA(Single Carrier Frequency Division Multiple Access), and the like. TheCDMA may be embodied with wireless (or radio) technology such as UTRA(Universal Terrestrial Radio Access) or CDMA2000. The TDMA may beembodied with wireless (or radio) technology such as GSM (Global Systemfor Mobile communications)/GPRS (General Packet Radio Service)/EDGE(Enhanced Data Rates for GSM Evolution). The OFDMA may be embodied withwireless (or radio) technology such as Institute of Electrical andElectronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE802-20, and E-UTRA (Evolved UTRA). The UTRA is a part of the UMTS(Universal Mobile Telecommunications System). The 3GPP (3rd GenerationPartnership Project) LTE (long term evolution) is a part of the E-UMTS(Evolved UMTS), which uses E-UTRA. The 3GPP LTE employs the OFDMA indownlink and employs the SC-FDMA in uplink. The LTE-Advanced (LTE-A) isan evolved version of the 3GPP LTE. WiMAX can be explained by an IEEE802.16e (WirelessMAN-OFDMA Reference System) and an advanced IEEE802.16m (WirelessMAN-OFDMA Advanced System). For clarity, the followingdescription focuses on the 3GPP LTE and LTE-A system. However, thetechnical spirit of the present invention is not limited thereto.

Hereinafter, the structure of a downlink radio frame will be describedwith reference to FIG. 1.

In a cellular Orthogonal Frequency Division Multiplexing (OFDM) radiopacket communication system, uplink/downlink data packet transmission isperformed in subframe units. One subframe is defined as a predeterminedtime interval including a plurality of OFDM symbols. The 3GPP LTEstandard supports a type 1 radio frame structure applicable to FrequencyDivision Duplex (FDD) and a type 2 radio frame structure applicable toTime division duplexing (TDD).

FIG. 1(a) is a diagram showing the structure of the type 1 radio frame.A downlink radio frame includes 10 subframes, and one subframe includestwo slots in a time domain. A time required to transmit one subframe isdefined in a Transmission Time Interval (TTI). For example, one subframemay have a length of 1 ms and one slot may have a length of 0.5 ms. Oneslot may include a plurality of OFDM symbols in a time domain andinclude a plurality of Resource Blocks (RBs) in a frequency domain.Since the 3GPP LTE system uses OFDMA in downlink, an OFDM symbolindicates one symbol duration. The OFDM symbol may be called a SC-FDMAsymbol or a symbol duration. A RB is a resource allocation unit andincludes a plurality of contiguous carriers in one slot.

The number of OFDM symbols included in one slot may be changed accordingto the configuration of a Cyclic Prefix (CP). The CP includes anextended CP and a normal CP. For example, if the OFDM symbols areconfigured by the normal CP, the number of OFDM symbols included in oneslot may be seven. If the OFDM symbols are configured by the extendedCP, the length of one OFDM symbol is increased, the number of OFDMsymbols included in one slot is less than that of the case of the normalCP. In case of the extended CP, for example, the number of OFDM symbolsincluded in one slot may be six. If a channel state is instable, forexample, if a User Equipment (UE) moves at a high speed, the extended CPmay be used in order to further reduce interference between symbols.

In case of using the normal CP, since one slot includes seven OFDMsymbols, one subframe includes 14 OFDM symbols. At this time, the firsttwo or three OFDM symbols of each subframe may be allocated to aPhysical Downlink Control Channel (PDCCH) and the remaining OFDM symbolsmay be allocated to a Physical Downlink Shared Channel (PDSCH).

The structure of a type 2 radio frame is shown in FIG. 1(b). The type 2radio frame includes two half-frames, each of which is made up of fivesubframes, a downlink pilot time slot (DwPTS), a guard period (GP), andan uplink pilot time slot (UpPTS), in which one subframe consists of twoslots. That is, one subframe is composed of two slots irrespective ofthe radio frame type. DwPTS is used to perform initial cell search,synchronization, or channel estimation. UpPTS is used to perform channelestimation of a base station and uplink transmission synchronization ofa user equipment (UE). The guard interval (GP) is located between anuplink and a downlink so as to remove interference generated in uplinkdue to multi-path delay of a downlink signal. That is, one subframe iscomposed of two slots irrespective of the radio frame type.

The structure of the radio frame is only exemplary. Accordingly, thenumber of subframes included in the radio frame, the number of slotsincluded in the subframe or the number of symbols included in the slotmay be changed in various manners.

FIG. 2 is a diagram showing an example of a resource grid in onedownlink slot. OFDM symbols are configured by the normal CP. Referringto FIG. 2, the downlink slot includes a plurality of OFDM symbols in atime domain and includes a plurality of RBs in a frequency domain.Although one downlink slot includes seven OFDM symbols and one RBincludes 12 subcarriers, the present invention is not limited thereto.Each element of the resource grid is referred to as a Resource Element(RE). For example, an RE a(k,l) is located at a k-th subcarrier and an1-th OFDM symbol. In case of the normal CP, one RB includes 12×7 REs (incase of the extended CP, one RB includes 12×6 REs). Since a distancebetween subcarriers is 15 kHz, one RB includes about 180 kHz in thefrequency region. N^(DL) denotes the number of RBs included in thedownlink slot. The N^(DL) is determined based on downlink transmissionbandwidth set by scheduling of a base station (BS).

FIG. 3 is a diagram showing the structure of a downlink subframe. Amaximum of three OFDM symbols of a front portion of a first slot withinone subframe corresponds to a control region to which a control channelis allocated. The remaining OFDM symbols correspond to a data region towhich a Physical Downlink Shared Channel (PDSCH) is allocated. The basicunit of transmission becomes one subframe. That is, a PDCCH and a PDSCHare allocated to two slots. Examples of the downlink control channelsused in the 3GPP LTE system include, for example, a Physical ControlFormat Indicator Channel (PCFICH), a Physical Downlink Control Channel(PDCCH), a Physical Hybrid automatic repeat request Indicator Channel(PHICH), etc. The PCFICH is transmitted at a first OFDM symbol of asubframe, and includes information about the number of OFDM symbols usedto transmit the control channel in the subframe. The PHICH includes aHARQ ACK/NACK signal as a response to uplink transmission. The controlinformation transmitted through the PDCCH is referred to as DownlinkControl Information (DCI). The DCI includes uplink or downlinkscheduling information or an uplink transmit power control command for acertain UE group. The PDCCH may include resource allocation andtransmission format of a Downlink Shared Channel (DL-SCH), resourceallocation information of an Uplink Shared Channel (UL-SCH), paginginformation of a Paging Channel (PCH), system information on the DL-SCH,resource allocation of a higher layer control message such as a RandomAccess Response (RAR) transmitted on the PDSCH, a set of transmit powercontrol commands for individual UEs in a certain UE group, transmitpower control information, activation of Voice over IP (VoIP), etc. Aplurality of PDCCHs may be transmitted within the control region. The UEmay monitor the plurality of PDCCHs. The PDCCHs are transmitted on anaggregation of one or several contiguous control channel elements(CCEs). The CCE is a logical allocation unit used to provide the PDCCHsat a coding rate based on the state of a radio channel. The CCEcorresponds to a plurality of resource element groups. The format of thePDCCH and the number of available bits are determined based on acorrelation between the number of CCEs and the coding rate provided bythe CCEs. The base station determines a PDCCH format according to a DCIto be transmitted to the UE, and attaches a Cyclic Redundancy Check(CRC) to control information. The CRC is masked with a Radio NetworkTemporary Identifier (RNTI) according to an owner or usage of the PDCCH.If the PDCCH is for a specific UE, a cell-RNTI (C-RNTI) of the UE may bemasked to the CRC. Alternatively, if the PDCCH is for a paging message,a paging indicator identifier P-RNTI) may be masked to the CRC. If thePDCCH is for system information (more specifically, a system informationblock (SIB)), a system information identifier and a system informationRNTI (SI-RNTI) may be masked to the CRC. To indicate a random accessresponse that is a response for transmission of a random access preambleof the UE, a random access-RNTI (RA-RNTI) may be masked to the CRC.

FIG. 4 is a diagram showing the structure of an uplink frame. The uplinksubframe may be divided into a control region and a data region in afrequency domain. A Physical Uplink Control Channel (PUCCH) includinguplink control information is allocated to the control region. APhysical uplink Shared Channel (PUSCH) including user data is allocatedto the data region. In order to maintain single carrier characteristics,one UE does not simultaneously transmit the PUCCH and the PUSCH. ThePUCCH for one UE is allocated to an RB pair in a subframe. RBs belongingto the RB pair occupy different subcarriers with respect to two slots.Thus, the RB pair allocated to the PUCCH is “frequency-hopped” at a slotedge.

Reference Signal

In the MIMO system, each transmitting antenna has an independent datachannel. The transmitting antenna may signify a virtual antenna or aphysical antenna. A reception subject may estimate a channel withrespect to each transmitting antenna of a transmission subject, therebybeing capable of receiving data transmitted from each transmittingantenna. Channel estimation refers to a process of recovering a receivedsignal by compensating for a distortion in a signal, which is caused byfading. Herein, fading refers to an effect wherein the intensity of asignal is changed abruptly due to a multi path-time delay in a wirelesstelecommunications system environment. In order to perform channelestimation, a reference signal commonly known by the transmissionsubject and the reception subject is required. A reference signal mayalso simply be referred to as an RS (Reference Signal) or may also bereferred to as a Pilot depending upon the applied standard.

In the legacy 3GPP LTE Release-8 or -9 system, a downlink referencesignal transmitted by a base station is defined. Downlink referencesignal is a pilot signal for coherent demodulation of such a channel asPDSCH (Physical Downlink Shared CHannel), PCFICH (Physical ControlFormat Indicator CHannel), PHICH (Physical Hybrid Indicator CHannel),PDCCH (Physical Downlink Control CHannel) and the like. The downlinkreference signal may be categorized into a common reference signal (CRS)shared by all user equipments in a cell and a dedicated reference signal(DRS) for a specific user equipment only. The common reference signalmay be called a cell-specific reference signal. And, the dedicatedreference signal may be called a user equipment-specific (UE-specific)reference signal or a demodulation reference signal (DMRS).

Downlink reference signal assignment in the legacy 3GPP LTE system isdescribed as follows. First of all, a position (i.e., a reference signalpattern) of a resource element for carrying a reference signal isdescribed with reference to one resource block pair (i.e., ‘one subframelength in time domain’×‘12-subcarrier length in frequency domain’). Asingle subframe is configured with 14 OFDM symbols (in case of a normalCP) or 12 OFDM symbols (in case of an extended CP). The number ofsubcarriers in a single OFDM symbol is set to one of 128, 256, 512,1024, 1536 and 2048 to use.

FIG. 5 shows a pattern of a common reference signal (CRS) in case that1-TTI (i.e., 1 subframe) has 14 OFDM symbols. FIG. 5 (a), FIG. 5 (b) andFIG. 5 (c) relates to a CRS pattern for a system having 1 Tx(transmitting) antenna, a CRS pattern for a system having 2 Tx antennasand a CRS pattern for a system having 4 Tx antennas, respectively.

In FIG. 5, R0 indicates a reference signal for an antenna port index 0.In FIG. 5, R1 indicates a reference signal for an antenna port index 1,R2 indicates a reference signal for an antenna port index 2, and R3indicates a reference signal for an antenna port index 3. Regarding aposition of an RE for carrying a reference signal for each of theantenna ports, no signal is transmitted from the rest of all antennaports except the antenna port for transmitting a reference signal toprevent interference.

FIG. 6 shows that a reference signal pattern is shifted in each cell toprevent reference signals of various cells from colliding with eachother. Assuming that a reference signal pattern for one antenna portshown in FIG. 5 (a) is used by a cell #1 (Cell 1) shown in FIG. 6, inorder to prevent collision of reference signals between cells includinga cell #2 adjacent to the cell #1, a cell #3 adjacent to the cell #1 andthe like, it is able to protect a reference signal in a manner ofshifting a reference signal pattern by subcarrier or OFDM symbol unit infrequency or time domain. For instance, in case of 1 Tx antennatransmission, since a reference signal is situated in 6-subcarrierinterval on a single OFDM symbol, if a shift by subcarrier unit infrequency domain is applied to each cell, at least 5 adjacent cells maybe able to situate reference signals on different resource elements,respectively. For instance, a frequency shift of a reference signal maybe represented as the cell #2 and the cell #6 in FIG. 6.

Moreover, by multiplying a downlink reference signal per cell by apseudo-random (PN) sequence and then transmitting the multiplied signal,interference caused to a receiver by a reference signal received from anadjacent cell can be reduced to enhance channel estimation performance.This PN sequence may be applicable by OFDM symbol unit in a singlesubframe. Regarding the PN sequence, a different sequence may beapplicable per cell ID, subframe number or OFDM symbol position.

In a system [e.g., 8-Tx antenna supportive wireless communication system(e.g., 3GPP LTE Release-10 system, a systems according to 3GPP LTEReleases next to Release-10, etc.)] having antenna configuration moreextended than a legacy 4-Tx antenna supportive communication system(e.g., 3GPP LTE Release-8 system, 3GPP LTE Release-9 system, etc.), DMRSbased data demodulation is taken into consideration to support efficientmanagement & operation and developed transmission scheme of referencesignals. In particular, in order to support data transmission viaextended antennas, it may be able to define DMRS for at least twolayers. Since DMRS is precoded by the same precoder of data, it is easyfor a receiving side to estimate channel information for demodulatingdata without separate precoding information. Meanwhile, a downlinkreceiving side is able to acquire channel information precoded for theextended antenna configuration through DMRS. Yet, a separate referencesignal other than the DMRS is requested to acquire non-precoded channelinformation. Hence, in a system by LTE-A standards, a reference signal(i.e., CSI-RS) for a receiving side to acquire channel state information(CSI) can be defined. In particular, CSI-RS may be transmitted via 8antenna ports. In order to discriminate a CSI-RS transmitted antennaport from an antenna port of 3GPP LTE Release-8/9, it may be able to useantenna port indexes 15 to 22.

Configuration of Downlink Control Channel

As a region for transmitting a downlink control channel, first threeOFDM symbols of each subframe are available. In particular, 1 to 3 OFDMsymbols are available in accordance with overhead of the downlinkcontrol channel. In order to adjust the number of OFDM symbols for adownlink control channel in each subframe, it may be able to use PCFICH.And, it is able to use PHICH to provide an acknowledgment response[ACK/NACK (acknowledgement/negative-acknowledgement)] to an uplinktransmission in downlink. Moreover, it is able to use PDCCH to transmitcontrol information for a downlink or uplink data transmission.

FIG. 7 and FIG. 8 show that the above-configured downlink controlchannels are assigned by resource element group (REG) unit in a controlregion of each subframe. FIG. 7 relates to a system having 1- or 2-Txantenna configuration and FIG. 8 relates to a system having 4-Tx antennaconfiguration. Referring to FIG. 7 and FIG. 8, REG corresponding to abasic resource unit for assigning a control channel is configured with 4contiguous Res in frequency domain except a resource element forassigning a reference signal. A specific number of REGs are availablefor a transmission of a downlink control channel in accordance withoverhead of the downlink control channel.

PCFICH (Physical Control Format Indicator Channel)

In order to provide every subframe with resource allocation informationof the corresponding subframe and the like, it is able to transmit PDCCHbetween OFDM symbol indexes 0 to 2. In accordance with overhead of acontrol channel, it may be able to use the OFDM symbol index 0, the OFDMsymbol indexes 0 and 1, or the OFDM symbol indexes 0 to 2. Thus, thenumber of OPFDM symbols used for a control channel is changeable foreach subframe. And, information on the OFDM symbol number may beprovided via PCFICH. Hence, the PCFICH should be transmitted in everysubframe.

Three kinds of informations can be provided through the PCFICH. Table 1in the following shows CFI (control format indicator) of PCFICH. ‘CFI=1’indicates that PDCCH is transmitted on OFDM symbol index 0, ‘CFI=2’indicates that PDCCH is transmitted on OFDM symbol indexes 0 and 1, and‘CFI=3’ indicates that PDCCH is transmitted on OFDM symbol indexes 0 to2.

TABLE 1 CFI codeword CFI <b₀, b₁, . . . , b₃₁> 1 <0, 1, 1, 0, 1, 1, 0,1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0,1> 2 <1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1,0, 1, 1, 0, 1, 1, 0, 1, 1, 0> 3 <1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1,1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1> 4 <0, 0, 0, 0,0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, (Reserved) 0, 0, 0,0, 0, 0, 0, 0, 0, 0, 0>

Information carried on PCFICH may be defined different in accordancewith a system bandwidth. For instance, in case that a bandwidth of asystem is smaller than a specific threshold, ‘CFI=1’ may indicate that 2OFDM symbols are used for PDCCH. ‘CFI=2’ may indicate that 3 OFDMsymbols are used for PDCCH. And, ‘CFI=3’ may indicate that 4 OFDMsymbols are used for PDCCH.

FIG. 9 is a diagram for a scheme of transmitting PCIFCH. REG shown inFIG. 9 is configured with 4 subcarriers, and more particularly, withdata subcarriers except RS (reference signal). Generally, a transmitdiversity scheme may apply thereto. A position of the REG may befrequency-shifted per cell (i.e., in accordance with a cell identifier)not to cause interference between cells. Additionally, PCFICH is alwaystransmitted on a 1^(st) OFDM symbol (i.e., OFDM symbol index 0) of asubframe. Hence, when a receiving end receives a subframe, the receivingend acquires the number of OFDM symbols for carrying PDCCH by checkinginformation of PCFICH and is then able to receive control informationtransmitted on the PDCCH.

PHICH (Physical Hybrid-ARQ Indicator Channel)

FIG. 10 is a diagram to illustrate positions of PCFICH and PHICHgenerally applied for a specific bandwidth. ACK/NACK information on anuplink data transmission is transmitted on PHICH. Several PHICH groupsare created in a single subframe and several PHICHs exist in a singlePHICH group. Hence, PHICH channels for several user equipments areincluded in the single PHICH group.

Referring to FIG. 10, PHICH assignment for each user equipment inseveral PHICH groups are performed using a lowest PRB (physical resourceblock) index of PUSCH resource allocation and a cyclic shift index for ademodulation reference signal (DMRS) transmitted on a UL (uplink) grantPDCCH. In this case, the DMRS is a UL reference signal and is the signalprovided together with a UL transmission for channel estimation fordemodulation of UL data. Moreover, PHICH resource is known through suchan index pair as (n_(PHICH) ^(group),n_(PHICH) ^(seq)). In (n_(PHICH)^(group),n_(PHICH) ^(seq)), n_(PHICH) ^(group) means a PHICH groupnumber) and n_(PHICH) ^(seq) means an orthogonal sequence index in thecorresponding PHICH group. n_(PHICH) ^(group) and n_(PHICH) ^(seq) isdefined as Formula 1.

n _(PHICH) ^(group)=(I _(PRB) _(_) _(RA) ^(lowest) ^(_) ^(index) +n_(DMRS))mod N _(PHICH) ^(group)

n _(PHICH) ^(seq)=(└I _(PRB) _(_) _(RA) ^(lowest) ^(_) ^(index) /N_(PHICH) ^(group) ┘+n _(DMRS))mod 2N _(SF) ^(PHICH)[Formula 1]

In Formula 1, n_(DMRS) indicates a cyclic shift of DMRS used for a PHICHassociated UL transmission. And, N_(SF) ^(PHICH) indicates a spreadingfactor size used for PHICH. I_(PRB) _(_) _(RA) ^(lowest) ^(_) ^(index)indicates a lowest PRB index of a UL resource allocation. N_(PHICH)^(group) indicates the number of the configured PHICH groups and may bedefined as Formula 2.

$\begin{matrix}{N_{PHICH}^{group} = \left\{ \begin{matrix}\left\lceil {N_{g}\left( {N_{RB}^{DL}/8} \right)} \right\rceil & {{for}\mspace{14mu} {normal}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}} \\{2 \cdot \left\lceil {N_{g}\left( {N_{RB}^{DL}/8} \right)} \right\rceil} & {{for}\mspace{14mu} {extended}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}\end{matrix} \right.} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

In Formula 2, N_(g) indicates an amount of PHICH resource transmitted onPBCH (Physical Broadcast Channel) and N_(g) is represented asN_(g)ε{⅙,½,1,2} in 2-bit size.

One example of an orthogonal sequence defined by the legacy 3GPP LTERelease-8/9 is shown in Table 2.

TABLE 2 Orthogonal sequence Sequence index Normal cyclic prefix Extendedcyclic prefix n_(PHICH) ^(seq) N_(SF) ^(PHICH) = 4 N_(SF) ^(PHICH) = 2 0[+1 +1 +1 +1] [+1 +1] 1 [+1 −1 +1 −1] [+1 −1] 2 [+1 +1 −1 −1] [+j +j] 3[+1 −1 −1 +1] [+j −j] 4 [+j +j +j +j] — 5 [+j −j +j −j] — 6 [+j +j −j−j] — 7 [+j −j −j +j] —

FIG. 11 is a diagram to illustrate a position of a downlink (DL)resource element having PHICH group mapped thereto. Referring to FIG.11, PHICH group may be configured in different time region (i.e., adifferent OS (OFDM symbol)) within a single subframe.

PDCCH (Physical Downlink Control Channel)

Control information transmitted on PDCCH may have control informationsize and usage differing in accordance with a DCI (downlink controlinformation) format. And, a size of the PDCCH may vary in accordancewith a coding rate. For instance, DCI formats used by the legacy 3GPPLTE Release-8/9 may be defined as Table 3.

TABLE 3 DCI format Objectives 0 Scheduling of PUSCH 1 Scheduling of onePDSCH codeword 1A Compact scheduling of one PDSCH codeword 1BClosed-loop single-rank transmission 1C Paging, RACH response anddynamic BCCH 1D MU-MIMO 2 Scheduling of rank-adapted closed-loop spatialmultiplexing mode 2A Scheduling of rank-adapted open-loop spatialmultiplexing mode 3 TPC commands for PUCCH and PUSCH with 2 bit poweradjustments 3A TPC commands for PUCCH and PUSCH with single bit poweradjustments

The DCI format shown in Table 3 is independently applied per userequipment and PDCCHs of several user equipments can be simultaneouslymultiplexed within a single subframe. The multiplexed PDCCH of each ofthe user equipments is independently channel-coded and CRC is appliedthereto. The CRC of the PDCCH is masked with a unique identifier of eachof the user equipments and can be applied to enable the correspondinguser equipment to receive the PDCCH of its own. Yet, since a userequipment is basically unable to know a position of its PDCCH channel,the user equipment checks whether each of the entire PDCCH channels ofthe corresponding DCI format matches the PDCCH channel having the ID ofthe corresponding user equipment for each subframe and needs to performblind detection until receiving the corresponding PDCCH. A basicresource allocation unit of the PDCCH is CCE (control channel element)and a single CCE is configured with 9 TEGs. A single PDCCH may beconfigured with 1, 2, 4 or 8 CCEs. PDCCH configured in accordance witheach user equipment is interleaved into a control channel region of eachsubframe and then mapped by a CCE-to-RE mapping rule. This may vary anRE position having a CCE mapped thereto in accordance with the OFDMsymbol number for a control channel of each subframe, the PHICH groupnumber, Tx antennas, a frequency shift and the like.

Uplink Retransmission

Uplink (UL) retransmission may be indicated via the aforementioned PHICHand the DCI format 0 (i.e., DCI format for scheduling PUSCHtransmission). A user equipment receives ACK/NACK for a previous ULtransmission via PHICH and is then able to perform a synchronousnon-adaptive retransmission. Alternatively, a user equipment receives aUL grant via DCI format 0 PDCCH from a base station and is then able toperform a synchronous adaptive retransmission.

The synchronous transmission means that a retransmission is performed ata predetermined timing point (e.g., (n+k)^(th) subframe) after a timingpoint (e.g., n^(th) subframe) of transmitting a data packet. In bothcases of the retransmission by PHICH and the retransmission by UL grantPDCCH, the synchronous retransmission is performed.

Regarding the non-adaptive retransmission of performing a retransmissionon PHICH, the same frequency resource and transmitting method of theformer frequency resource (e.g., physical resource block (PRB) region)and transmitting method (e.g., modulation scheme, etc.) used for aprevious transmission are applied to the retransmission. Meanwhile,regarding the adaptive retransmission of performing a retransmission viaUL grant PDCCH, a frequency resource and transmitting method forperforming a retransmission may be set different from those of aprevious transmission in accordance with a scheduling controlinformation indicated by a UL grant.

In case that a user equipment receives a UL grant PDCCH as soon asreceives PHICH, the user equipment may be able to perform a ULtransmission in accordance with control information of the UL grantPDCCH by ignoring the PHICH. Since a new data indicator (NDI) isincluded in the UL grant PDCCH (e.g., DCI format 0), if NDI bit istoggled more than a previously provided NDI value, the user equipmentregards a previous transmission as successful and is then able totransmit new data. Meanwhile, although the user equipment receives ACKfor a previous transmission via PHICH, unless the NDI value is toggledin the UL grant PDCCH received simultaneously with or after the PHICHreception, the user equipment is configured not to flush a buffer forthe previous transmission.

Uplink Transmission Configuration

FIG. 12 is a diagram for a structure of a transmitter by SC-FDMA.

First of all, a single block configured with N symbols inputted to atransmitter is converted to a parallel signal via a serial-to-parallelconverter 1201. The parallel signal spreads via an N-point DFT module1202. The spreading signal is mapped to a frequency region via asubcarrier mapping module 1203. Signals on subcarriers configure linearcombination of N symbols. The signal mapped to the frequency region istransformed into a time-domain signal via an M-point IFFT module 1204.The time-domain signal is converted to a parallel signal via aparallel-to-serial converter 1205 and then has a CP added thereto. Theeffect of the IFFT processing by the M-point IFFT module 1204 iscancelled out by the DFT processing of the N-point DFT module 1202 tosome extent. In this point, the SC-FDMA may be named DFT-s-OFDMA(DFT-spread-OFDMA). Moreover, although the signal inputted to the DFTmodule 1202 has a low PAPR (peak-to-average power ratio) or CM (cubicmetric), it may have a high PAPR after DFT processing. And, the signaloutputted by the IFFT processing of the IFFT module 1204 may have a lowPAPR again. In particular, according to the SC-FDMA, transmission isperformed by avoiding a non-linear distortion interval of a poweramplifier (PA), whereby a cost for implementation of a transmitting endcan be reduced.

FIG. 13 is a diagram to describe a scheme of mapping a signal outputtedfrom the DFT module 1202 to a frequency region. By performing one of thetwo schemes shown in FIG. 13, a signal outputted from an SC-FDMAtransmitter can meet the single carrier property. FIG. 13 (a) shows alocalized mapping scheme of locally mapping signals outputted from theDFT module 1202 to a specific part of a subcarrier region. FIG. 13 (b)shows a distributed mapping scheme of mapping signals outputted from theDFT module 1202 to a whole subcarrier region by being distributed. Inthe legacy 3GPP LTE Release-8/9 system, it is defined that the localizedmapping scheme is used.

FIG. 14 is a block diagram to describe a transmission processing of areference signal to demodulate a transmitted signal by SC-FDMA. In thelegacy 3GPP LTE Release-8/9 system, a data part is transmitted in amanner of transforming a signal generated from a time domain into afrequency-domain signal by DFT processing, performing subcarrier mappingon the frequency-domain signal, and then performing IFFT processing onthe mapped signal [cf. FIG. 12]. Yet, a reference signal (RS) is definedas directly generated in frequency domain without DFT processing, mappedto subcarrier, undergoing IFFT processing, and then having CP additionthereto.

FIG. 15 is a diagram to illustrate a symbol position having a referencesignal (RS) mapped thereto in a subframe structure according to SC-FDMA.FIG. 15 (a) shows that RS is located at 4^(th) SC-FDMA symbol of each of2 slots in a single subframe in case of a normal CP. FIG. 15 (b) showsthat RS is located at 3^(rd) SC-FDMA symbol of each of 2 slots in asingle subframe in case of an extended CP.

Clustered DFT-s-OFDMA scheme is described with reference to FIGS. 16 to19 as follows. The clustered DFT-s-OFDMA is the modification of theaforementioned SC-FDMA. According to the clustered DFT-s-OFDMA, aDFT-processed signal is segmented into a plurality of sub-blocks andthen mapped to a spaced position in frequency domain.

FIG. 16 is a diagram to describe a clustered DFT-s-OFDMA scheme on asingle carrier. For instance, a DFT output may be partitioned into Nsbsub-blocks (sub-blocks #0 to #Nsb−1). When sub-blocks are mapped to afrequency region, the sub-blocks #0 to #NSb−1 are mapped to a singlecarrier (e.g., carrier of 20 MHz bandwidth, etc.) and each of thesub-blocks may be mapped to a position spaced in the frequency region.And, each of the sub-blocks may be locally mapped to the frequencyregion.

FIG. 17 and FIG. 18 are diagrams to describe a clustered DFT-s-OFDMAscheme on multiple carriers.

In a situation (i.e., frequency bands of multiple carriers (cells) arecontiguously assigned) that multiple carriers (or multiple cells) areconfigured contiguously, if a subcarrier interval between contiguouscarriers is aligned, FIG. 17 is a diagram for one example that a signalcan be generated through a single IFFT module. For instance, a DFToutput may be segmented into Nsb sub-blocks (sub-blocks #0 to #NSb−1).In mapping sub-blocks to a frequency region, the sub-blocks #0 to #NSb−1can be mapped to component carriers #0 to #NSb−1, respectively [e.g.,each carrier (or cell) may have a bandwidth of 20 MHz]. moreover, eachof the sub-blocks may be mapped to a frequency region by beinglocalized. And, the sub-blocks mapped to the carriers (or cells) may betransformed into a time-domain signal through a single IFFT module.

In a situation (i.e., frequency bands of multiple carriers (cells) arenon-contiguously assigned) that multiple carriers (or multiple cells)are configured non-contiguously, FIG. 18 is a diagram for one examplethat a signal is generated using a plurality of IFFT modules. Forinstance, a DFT output may be segmented into Nsb sub-blocks (sub-blocks#0 to #NSb−1). In mapping sub-blocks to a frequency region, thesub-blocks #0 to #NSb−1 can be mapped to carriers #0 to #NSb−1,respectively [e.g., each carrier (or cell) may have a bandwidth of 20MHz]. moreover, each of the sub-blocks may be mapped to a frequencyregion by being localized. And, the sub-blocks mapped to the carriers(or cells) may be transformed into a time-domain signal through the IFFTmodules, respectively.

If the clustered DFT-s-OFDMA on the single carrier mentioned withreference to FIG. 16 is called intra-carrier (or intra-cell)DFT-s-OFDMA, the DFT-s-OFDMA on the multiple carriers (or cells)mentioned with reference to FIG. 17 or FIG. 18 may be calledinter-carrier (or inter-cell) DFT-s-OFDMA. Thus, the intra-carrierDFT-s-OFDMA and the inter-carrier DFT-s-OFDMA may be interchangeablyusable.

FIG. 19 is a diagram to describe a chuck-specific DFT-s-OFDMA scheme ofperforming DFT processing, frequency domain mapping and IFFT processingby chunk unit. The chunk-specific DFT-s-OFDMA may be called Nx SC-FDMA.A code block segmented signal is chunk-segmented into parts and channelcoding and modulation is performed on each of the parts. The modulatedsignal undergoes the DFT processing, the frequency domain mapping andthe IFFT processing in the same manner described with reference to FIG.12, outputs from the respective IFFTs are added up, and CP may be addedthereto. The Nx SC-FDMA scheme mentioned with reference to FIG. 19 maybe applicable to a contiguous multi-carrier (or multi-cell) case and anon-contiguous multi-carrier (or multi-cell) case both.

Structure of MIMO System

FIG. 20 is a diagram to illustrate a basic structure of MIMO systemhaving multiple Tx antennas and/or multiple Rx (receiving) antennas.Each block shown in FIG. 20 conceptionally indicates a function oroperation in a transmitting/receiving end for MIMO transmission.

A channel encoder shown in FIG. 20 indicates an operation of attaching aredundancy bit to an input data bit, whereby effect of noise and thelike from a channel can be reduced. A mapper indicates an operation ofconverting data bit information to data symbol information. Aserial-to-parallel converter indicates an operation of converting serialdata to parallel data. A multi-antenna encoder indicates an operation oftransforming a data symbol into a time-spatial signal. A multi-antennaof a transmitting end plays a role in transmitting this time-spatialsignal on a channel, while a multi-antenna of a receiving end plays arole in receiving the signal on the channel.

A multi-antenna decoder shown in FIG. 20 indicates an operation oftransforming the received time-spatial signal into each data symbol. Aparallel-to-serial converter indicates an operation of converting aparallel signal to a serial signal. A demapper indicates an operation oftransforming a data symbol to a bit information. A decoding operationfor a channel code is performed by a channel decoder, whereby data canbe estimated.

The MIMO transceiving system mentioned in the above description may havea single or several codewords spatially in accordance with a spacemultiplexing ratio. A case of having a single codeword spatially iscalled a single codeword (SCW) structure. And, a case of having severalcodewords is called a multiple codeword (MCW) structure.

FIG. 21 (a) is a block diagram to represent functionality of atransmitting end of an MIMO system having the SCW structure. And, FIG.21 (b) is a block diagram to represent functionality of a transmittingend of an MIMO system having the MCW structure.

Codebook Based Precoding Scheme

In order to support multi-antenna transmission, it may be able to applyprecoding of appropriately distributing transmission information to eachantenna in accordance with a channel status and the like. A codebookbased precoding scheme means the scheme performed in a following manner.First of all, a set of precoding matrixes is determined in atransmitting end and a receiving end. Secondly, the transmitting endmeasures channel information from the transmitting end and then feedsback information (i.e., a precoding matrix index (PMI)) indicating whatis a most appropriate precoding matrix to the transmitting end. Finally,the transmitting end applies an appropriate precoding to a signaltransmission based on the PMI. Since the appropriate precoding matrix isselected from the previously determined precoding matrix set, althoughan optimal precoding is not always applied, this is more advantageousthan the explicit feedback of optimal precoding information actuallycarried on channel information in reducing feedback overhead.

FIG. 22 is a diagram to describe basic concept of codebook basedprecoding.

According to a codebook based precoding scheme, a transmitting and areceiving end share codebook information including a prescribed numberof precoding matrixes in accordance with a transmission rank, the numberof antennas and the like. In particular, in case that feedbackinformation is finite, it is able to use the precoding based codebookscheme. The receiving end measures a channel status via a receivedsignal and is then able to deed back information (i.e., indexes of thecorresponding precoding matrixes) on the finite number of preferredprecoding matrixes based on the above-mentioned codebook information tothe transmitting end. For instance, the receiving end is able to selectan optimal precoding matrix in a manner of measuring a received signalby ML (maximum likelihood) or MMSE (minimum mean square error) scheme.FIG. 22 shows that the receiving end transmits the precoding matrixinformation per codeword to the transmitting end, by which the presentinvention may be non-limited.

Having received the feedback information from the receiving end, thetransmitting end may be able to select a specific precoding matrix fromthe codebook based on the received information. Having selected theprecoding matrix, the transmitting end performs a precoding in a mannerof multiplying layer signals, of which number corresponds to thetransmission rank, by the selected precoding matrix and may be then ableto transmit a precoded transmission signal via a plurality of antennas.In the precoding matrix, the number of rows is equal to that of theantennas and the number of columns is equal to a rank value. Since therank value is equal to the number of the layers, the number of thecolumns is equal to the number of layers. For instance, if the number ofthe Tx antennas and the number of the transmission layers are 4 and 2,respectively, the precoding matrix can be configured with 4×2 matrix.Information transmitted via each layer can be mapped to each antenna viathe precoding matrix.

Having received the signal precoded and transmitted by the transmittingend, the receiving end is able to reconstruct the received signal byperforming a processing inverse to that of the precoding performed bythe transmitting end. Generally, since the precoding matrix meets such aunitary matrix (U) condition as U*U^(H)=I, the inverse processing of theprecoding may be performed in a manner of multiplying the receivedsignal by Hermit matrix (P^(H)) of the precoding matrix (P) used for theprecoding of the transmitting end.

For instance, Table 4 in the following indicates a codebook used for adownlink transmission using 2 Tx antennas in 3GPP LTE Release-8/9 andTable 5 indicates a codebook used for a downlink transmission using 4 Txantennas in 3GPP LTE Release-8/9.

TABLE 4 Codebook Number of rank index 1 2 0$\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\1\end{bmatrix}$ $\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}$ 1 $\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- 1}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}$ 2 $\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\j\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}$ 3 $\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- j}\end{bmatrix}$ —

TABLE 5 Codebook Number of layers u index u_(n) 1 2 3 4 0 u₀ = [1 −1 −1−1]^(T) W₀ ^({1}) W₀ ^({14})/{square root over (2)} W₀ ^({124})/{squareroot over (3)} W₀ ^({1234})/2 1 u₁ = [1 −j 1 j]^(T) W₁ ^({1}) W₁^({12})/{square root over (2)} W₁ ^({123})/{square root over (3)} W₁^({1234})/2 2 u₂ = [1 1 −1 1]^(T) W₂ ^({1}) W₂ ^({12})/{square root over(2)} W₂ ^({123})/{square root over (3)} W₂ ^({3214})/2 3 u₃ = [1 j 1−j]^(T) W₃ ^({1}) W₃ ^({12})/{square root over (2)} W₃ ^({123})/{squareroot over (3)} W₃ ^({3214})/2 4 u₄ = [1 (−1 − j)/{square root over (2)}−j (1 − j)/{square root over (2)} ]^(T) W₄ ^({1}) W₄ ^({14})/{squareroot over (2)} W₄ ^({124})/{square root over (3)} W₄ ^({1234})/2 5 u₅ =[1 (1 − j)/{square root over (2)} j (−1 − j)/{square root over (2)}]^(T)W₅ ^({1}) W₅ ^({14})/{square root over (2)} W₅ ^({124})/{square rootover (3)} W₅ ^({1234})/2 6 u₆ = [1 (1 + j)/{square root over (2)} −j(−1 + j)/{square root over (2)}]^(T) W₆ ^({1}) W₆ ^({13})/{square rootover (2)} W₆ ^({134})/{square root over (3)} W₆ ^({1324})/2 7 u₇ = [1(−1 + j)/{square root over (2)} j (1 + j)/{square root over (2)}]^(T) W₇^({1}) W₇ ^({13})/{square root over (2)} W₇ ^({134})/{square root over(3)} W₇ ^({1324})/2 8 u₈ = [1 −1 1 1]^(T) W₈ ^({1}) W₈ ^({12})/{squareroot over (2)} W₈ ^({124})/{square root over (3)} W₈ ^({1234})/2 9 u₉ =[1 −j −1 −j]^(T) W₉ ^({1}) W₉ ^({14})/{square root over (2)} W₉^({134})/{square root over (3)} W₉ ^({1234})/2 10 u₁₀ = [1 1 1 −1]^(T)W₁₀ ^({1}) W₁₀ ^({13})/{square root over (2)} W₁₀ ^({123})/{square rootover (3)} W₁₀ ^({1324})/2 11 u₁₁ = [1 j −1 j]^(T) W₁₁ ^({1}) W₁₁^({13})/{square root over (2)} W₁₁ ^({134})/{square root over (3)} W₁₁^({1324})/2 12 u₁₂ = [1 −1 −1 1]^(T) W₁₂ ^({1}) W₁₂ ^({12})/{square rootover (2)} W₁₂ ^({123})/{square root over (3)} W₁₂ ^({1234})/2 13 u₁₃ =[1 −1 1 −1]^(T) W₁₃ ^({1}) W₁₃ ^({13})/{square root over (2)} W₁₃^({123})/{square root over (3)} W₁₃ ^({1324})/2 14 u₁₄ = [1 1 −1 −1]^(T)W₁₄ ^({1}) W₁₄ ^({13})/{square root over (2)} W₁₄ ^({123})/{square rootover (3)} W₁₄ ^({3214})/2 15 u₁₅ = [1 1 1 1]^(T) W₁₅ ^({1}) W₁₅^({12})/{square root over (2)} W₁₅ ^({123})/{square root over (3)} W₁₅^({1234})/2

In Table 5, W_(n) ^({s}) is obtained from a set {S} configured from aformula expressed as W_(n)=I−2u_(n)u_(n) ^(H)/u_(n) ^(H)u_(n). In thiscase, I indicates 4×4 unitary matrix and u_(n) indicates a value givenby Table 5.

Referring to Table 4, when a codebook for 2 Tx antennas has total 7precoding vectors/matrixes, since a unitary matrix is provided for anopen-loop system, there are total 6 precoding vectors/matrixes for theprecoding of a closed-loop system. Moreover, referring to Table 5, acodebook for 4 Tx antennas has total 64 precoding vectors/matrixes.

The above-mentioned codebooks have such a common property as a constantmodulus (CM) property, a nested property, a constrained alphabetproperty and the like. According to the CM property, each element ofevery precoding matrix within a codebook does not contain ‘0’ and isconfigured to have the same size. According to the nested property, aprecoding matrix of a low rank is designed to be configured with asubset of a specific column of a precoding matrix of a high rank.According to the constrained alphabet property, each element of everyprecoding matrix within a codebook is constrained. For instance, eachelement of a precoding matrix is limited only to an element (±1) usedfor BPSH (binary phase shift keying), elements (±1,±j) used for QPSK(quadrature phase shift keying), or elements

$\left( {{\pm 1},{\pm j},{\pm \frac{\left( {1 + j} \right)}{\sqrt{2}}},{\pm \frac{\left( {{- 1} + j} \right)}{\sqrt{2}}}} \right)$

used for 8-PSK. In the example of the codebook shown in Table 5, sincealphabet of each element of every precoding matrix within the codebookis configured with

$\left\{ {{\pm 1},{\pm j},{\pm \frac{\left( {1 + j} \right)}{\sqrt{2}}},{\pm \frac{\left( {{- 1} + j} \right)}{\sqrt{2}}}} \right\},$

it may be represented as having the constrained alphabet property.

Feedback Channel Structure

Basically, since a base station is unable to know information on a DLchannel in FDD system, channel information fed back by a user equipmentis used for a DL transmission. In case of the legacy 3GPP LTERelease-8/9 system, it is able to feed back DL channel information viaPUCCH or PUSCH. In case of the PUCCH, channel information isperiodically fed back. In case of the PUSCH, channel information isaperiodically fed back in accordance with a request made by a basestation. Moreover, feedback of channel information may be performed in amanner of feeding back the channel information on a whole frequency band(i.e., wideband (WB)) or the channel information on a specific number ofRBs (i.e., subband (SB)).

Extended Antenna Configuration

FIG. 23 shows examples of configuration of 8 Tx (transmitting) antennas.

FIG. 23 (a) shows a case that N antennas configure independent channelswithout being grouped, which is generally called ULA (uniform lineararray). In case that the number of antennas is small, the ULAconfiguration may be available. In case that the number of antennas isbig, multiple antennas are arranged in a manner being spaced apart fromeach other. Hence, it may be insufficient for a space of a transmitterand/or receiver to configure independent channels.

FIG. 23 (b) shows an antenna configuration (i.e., paired ULA) of ULAtype in which 2 antennas form a pair. In this case, an associatedchannel is established between a pair of the antennas and may beindependent from that of antennas of another pair.

Meanwhile, unlike the fact that the legacy 3GPP LTE Release-8/9 systemuses 4 Tx antennas in DL, the 3GPP LTE Release-10 system is able to use8 Tx antennas in DL. In order to apply this extended antennaconfiguration, it is necessary to install several Tx antennas ininsufficient space. Thus, the ULA antenna configuration shown FIG. 23(a) or FIG. 23 (b) may become inappropriate. Therefore, it may be ableto consider applying a dual-polarized (or cross-polarized) antennaconfiguration shown in FIG. 23 (c). In case of this configuration of Txantennas, even if a distance ‘d’ between antennas is relatively short,it is able to configure independent channels by lowering correlation.Therefore, data transmission of high throughput can be achieved.

Codebook Structures

If a transmitting end and a receiving end share a pre-defined codebookwith each other, it is able to lower an overhead for the receiving endto feedback precoding information to be used for MIMO transmission fromthe transmitting end. Hence, it is able to apply efficient precoding.

For one example of configuring a pre-defined codebook, it is able toconfigure a precoder matrix using DFT (discrete Fourier transform)matrix or Walsh matrix. Alternatively, it is able to configure precodersof various types by combination with a phase shift matrix, a shiftdiversity matrix or the like.

In configuring a DFT matrix based codebook, n×n DFT matrix can bedefined as Formula 3.

$\begin{matrix}{{{{DFTn}\text{:}\mspace{14mu} {D_{n}\left( {k,} \right)}} = {\frac{1}{\sqrt{n}}{\exp \left( {{- {j2\pi}}\; k\; \text{/}n} \right)}}},k,{ = 0},1,\ldots \mspace{11mu},{n - 1}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack\end{matrix}$

In the DFT matrix of Formula 3, a single matrix exists for a specificsize n only. Hence, in order to appropriately define and use variouskinds of precoding matrixes in accordance with a situation, it may beable to consider configuring to use a rotated version of the DFTn matrixin addition. One example of the rotated DFTn matrix is shown in Formula4.

$\begin{matrix}{{{{rotated}\mspace{14mu} {DFTn}\text{:}\mspace{14mu} {D_{n}^{({G,g})}\left( {k,} \right)}} = {\frac{1}{\sqrt{n}}{\exp \left( {{- {j2\pi}}\; {k\left( { + {g\text{/}G}} \right)}\text{/}n} \right)}}},k,{ = 0},1,\ldots \mspace{11mu},{n - 1},{g = 0},1,\ldots \mspace{11mu},{G.}} & \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack\end{matrix}$

In case that the DFT matrix shown in Formula 4 is configured, it is ableto create G rotated DFTn matrixes. And, the created matrixes may meetthe property of the DFT matrix.

In the following description, a householder-based codebook structure isexplained. The householder-based codebook structure may mean thecodebook configured with householder matrix. In particular, thehouseholder matrix is the matrix used for householder transform. Thehouseholder transform is a sort of linear transformation and may beusable in performing QR decomposition. The QR decomposition may meanthat a prescribed matrix is decomposed into an orthogonal matrix (Q) andan upper triangular matrix (R). The upper triangular matrix means asquare matrix of which components below main diagonal components are allzeros. One example of the 4×4 householder matrix is shown in Formula 5.

$\begin{matrix}{{M_{1} = {{I_{4} - {2u_{0}{u_{1}^{H}/{u_{0}}^{2}}}} = {\frac{1}{\sqrt{4}}*\begin{bmatrix}1 & 1 & 1 & 1 \\1 & 1 & {- 1} & {- 1} \\1 & {- 1} & 1 & {- 1} \\1 & {- 1} & {- 1} & 1\end{bmatrix}}}},{u_{0}^{T} = \begin{bmatrix}1 & {- 1} & {- 1} & {- 1}\end{bmatrix}}} & \left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack\end{matrix}$

It is able to create 4×4 unitary matrix having the CM property byhouseholder transformation. Like the codebook for the 4 Tx antennasshown in Table 5, n×n precoding matrix is created using the householdertransformation and a column subset of the created precoding matrix canbe configured to be used as a precoding matrix for a transmission of arank smaller than n.

Multi-Codebook Based Precoder Creation

A precoding operation used for a multi-antenna transmission may beexplained as an operation of mapping a signal transmitted via layer(s)to antenna(s). In particular, by X×Y precoding matrix, Y transmissionlayers (or streams) can be mapped to X Tx antennas.

In order to configure N_(t)×R precoding matrix used in transmitting Rstreams (i.e., Rank R) via N_(t) Tx antennas, a transmitting endreceives a feedback of at least one precoding matrix index (PMI) from areceiving end and is then able to configure a precoder matrix. Formula 6shows one example of a codebook configured with n_(c) matrixes.

P _(N) _(t) _(×R)(k)ε{P ₁ ^(N) ^(t) ^(×R) ,P ₂ ^(N) ^(t) ^(×R) ,P ₃ ^(N)^(t) ^(×R) , . . . ,P _(n) _(c) ^(N) ^(t) ^(×R)}  [Formula 6]

In Formula 6, k indicates a specific resource index (e.g., a subcarrierindex, a virtual resource index, a subband index, etc.). Formula 6 maybe configured in form of Formula 7.

$\begin{matrix}{{{P_{N_{t}R}(k)} = \begin{bmatrix}P_{{M_{t}R},1} \\P_{{M_{t}R},2}\end{bmatrix}},{N_{t} = {2 \cdot M_{t}}}} & \left\lbrack {{Formula}\mspace{14mu} 7} \right\rbrack\end{matrix}$

In Formula 7, P_(M) _(t) _(×R,2) may be configured in form of shiftingP_(M) _(t) _(×R,1) by a specific complex weight w₂. Hence, if adifference between P_(M) _(t) _(×R,1) and P_(M) _(t) _(×R,2) isrepresented as a specific complex weight, it may be expressed as Formula8.

$\begin{matrix}{{P_{N_{t}R}(k)} = \begin{bmatrix}{w_{1} \cdot P_{{M_{t}R},1}} \\{w_{2} \cdot P_{{M_{t}R},1}}\end{bmatrix}} & \left\lbrack {{formula}\mspace{14mu} 8} \right\rbrack\end{matrix}$

Moreover, Formula 8 may be represented as Formula 9 using Kronekerproduct (

).

$\begin{matrix}{{P_{{N_{t}R},n,m}(k)} = {{\begin{bmatrix}w_{1} \\w_{2}\end{bmatrix} \otimes P_{{M_{t}R},1}} = {W_{n} \otimes P_{m}}}} & \left\lbrack {{Formula}\mspace{14mu} 9} \right\rbrack\end{matrix}$

Kroneker product is an operation for 2 matrixes in random size. As aresult of Kroneker product operation, it is able to obtain a blockmatrix. For instance, Kroneker product of an m×n matrix A and a p×qmatrix B (i.e., A

B) may be represented as Formula 10. In Formula 10, a_(mn) indicates anelement of the matrix A and b_(pq) indicates an element of the matrix B.

${\mspace{166mu} {\left\lbrack {{Formula}\mspace{14mu} 10} \right\rbrack {{A \otimes B} =}}\quad}{\quad\begin{bmatrix}{a_{11}b_{11}} & {a_{11}b_{12}} & \ldots & {a_{11}b_{1q}} & \ldots & \ldots & {a_{1n}b_{11}} & {a_{1n}b_{12}} & \ldots & {a_{1n}b_{1q}} \\{a_{11}b_{21}} & {a_{11}b_{22}} & \ldots & {a_{11}b_{2q}} & \ldots & \ldots & {a_{1n}b_{21}} & {a_{1n}b_{22}} & \ldots & {a_{1n}b_{2q}} \\\vdots & \vdots & \ddots & \vdots & \; & \; & \vdots & \vdots & \ddots & \vdots \\{a_{11}b_{p\; 1}} & {a_{11}b_{p\; 2}} & \ldots & {a_{11}b_{pq}} & \ldots & \ldots & {a_{1n}b_{p\; 1}} & {a_{1n}b_{p\; 2}} & \ldots & {a_{1n}b_{pq}} \\\vdots & \vdots & \; & \vdots & \ddots & \; & \vdots & \vdots & \; & \vdots \\\vdots & \vdots & \; & \vdots & \; & \ddots & \vdots & \vdots & \; & \vdots \\{a_{m\; 1}b_{11}} & {a_{m\; 1}b_{12}} & \ldots & {a_{m\; 1}b_{1q}} & \ldots & \ldots & {a_{mn}b_{11}} & {a_{mn}b_{12}} & \ldots & {a_{mn}b_{1q}} \\{a_{m\; 1}b_{21}} & {a_{m\; 1}b_{22}} & \ldots & {a_{m\; 1}b_{2q}} & \ldots & \ldots & {a_{mn}b_{21}} & {a_{mn}b_{22}} & \ldots & {a_{mn}b_{2q}} \\\vdots & \vdots & \ddots & \vdots & \; & \; & \vdots & \vdots & \ddots & \vdots \\{a_{m\; 1}b_{p\; 1}} & {a_{m\; 1}b_{p\; 2}} & \ldots & {a_{m\; 1}b_{pq}} & \ldots & \ldots & {a_{mn}b_{p\; 1}} & {a_{mn}b_{p\; 2}} & \ldots & {a_{mn}b_{pq}}\end{bmatrix}}$

A partial matrix

$\quad\begin{bmatrix}w_{1} \\w_{2}\end{bmatrix}$

of precoding and P_(M) _(t) _(×R,1) in Formula 9 may be independentlyfed back from a receiving end. And, a transmitting end is able toconfigure and use a precoder like Formula 8 or Formula 9 using eachfeedback information. In case of applying the form of Formula 8 orFormula 9, W is always configured in form of 2×1 vector and may beconfigured in form of a codebook shown I Formula 11.

$\begin{matrix}{{W \in \begin{bmatrix}1 \\^{j\frac{2\pi}{N}i}\end{bmatrix}},{i = 0},\ldots \mspace{11mu},{N - 1}} & \left\lbrack {{Formula}\mspace{14mu} 11} \right\rbrack\end{matrix}$

In Formula 11, N indicates the total number of precoding vectorscontained in the codebook and i may be used as an index of a vector. Inorder to obtain proper performance by minimizing feedback overhead, imay be usable by being set to 2, 4 or 8. Moreover, P_(M) _(t) _(×R,1)may be configured as a codebook for 4 Tx antennas or a codebook for 2 Txantennas. For this, the codebook of Table 4 or Table 5 (e.g., thecodebook for 2 or 4 Tx antennas defined in 3GPP LTE Release-8/9) isusable. And, P_(M) _(t) _(×R,1) may be configured in rotated DFT form aswell.

Moreover, a matrix W may be available in form of 2×2 matrix. Formula 12shows one example of the 2×2 matrix W.

$\begin{matrix}{{{P_{{{N_{t}2}R},n,m}(k)} = {{\begin{bmatrix}w_{1} & w_{3} \\w_{2} & w_{4}\end{bmatrix} \otimes P_{{M_{t}R},1}} = {W_{n} \otimes P_{m}}}},{N_{t} = {2 \cdot M_{t}}}} & \left\lbrack {{Formula}\mspace{14mu} 12} \right\rbrack\end{matrix}$

In case of the configuration of Formula 12, if a maximum rank of thecodebook P_(M) _(t) _(×R,1) is R, it may be able to design a codebook ofa rank up to 2R. For instance, in case of using the codebook shown inTable 4 as P_(M) _(t) _(×R,1), according to Formula 9, it may be usableup to 4 (R=4) as a maximum rank. On the other hand, according to Formula12, it may be able to use a maximum rank up to 8 (2R=8). Hence, in thesystem configured with 8 Tx antennas, it is able to configure a precodercapable of 8×8 transmission. In this case, W may be configured in formof a codebook shown in Formula 13.

$\begin{matrix}{{W \in \begin{bmatrix}1 & 1 \\^{j\frac{2\pi}{N}i} & {- ^{j\frac{2\pi}{N}i}}\end{bmatrix}},{i = 0},\ldots \mspace{11mu},{N - 1}} & \left\lbrack {{Formula}\mspace{14mu} 13} \right\rbrack\end{matrix}$

The precoder configuring method according to Formula 9 or Formula 12 mayapply differently in accordance with each rank. For instance, the methodof Formula 9 is used for a case of a rank equal to or lower than 4(R≦4). And, the method of Formula 12 may be used for a case of a rankequal to or higher than 5 (R≧5). Alternatively, the method of Formula 9is used only for a case of a rank 1 (R=1). In other cases (i.e., rank 2or higher (R≧2)), it may be able to use the method of Formula 12. The Wand P mentioned in association with Formula 9 and Formula 12 may be fedback to have the property as shown in Table 6.

TABLE 6 Case W/P Frequency One of two matrixes may be configured to befed back on subband and the granularity 1 other may be configured to befed back on wideband. Frequency One of two matrixes may be configured tobe fed back on nest-M band and the granularity 2 other may be configuredto be fed back on wideband. Time One of two matrixes may be configuredto be fed back by periods N and the granularity other may be configuredto be fed back by periods M. Feedback One of two matrixes may beconfigured to be fed back on PUSCH and the channel 1 other may beconfigured to be fed back on PUCCH. Feedback In case of feedback onPUSCH, one (e.g., W) of two matrixes may be channel 2 configured to befed back on subband and the other (e.g., P) may be configured to be fedback on wideband. In case of feedback on PUCCH, both Q and P may beconfigured to be fed back on wideband. Unequal One (e.g., P) of twomatrixes may be configured to be coded at a more reliable protectionrating rate and the other (e.g., W) may be configured to be coded at arelatively low coding rate. Alphabet Alphabet of a matrix W may beconfigured to be constrained by BPSK and restriction 1 alphabet of amatrix P may be configured to be constrained by QPSK or 8 PSK. AlphabetAlphabet of a matrix W may be configured to be constrained by QPSK andrestriction 2 alphabet of a matrix P may be configured to be constrainedby QPSK or 8 PSK.

In the following description, a multi-codebook based precoder having thenested property is explained.

First of all, it is able to configure a codebook using the method ofFormula 9 and the method of Formula 12 appropriately. Yet, in somecases, it may be impossible to configure a precoder unless using twokinds of combinations. To solve this problem, it may be able toconfigure and use a precoder as shown in Formula 14.

$\begin{matrix}{{P_{{N_{t}N_{t}},n,m} = {{\begin{bmatrix}w_{1} & w_{3} \\w_{2} & w_{4}\end{bmatrix} \otimes P_{M_{t}M_{t}}} = {W_{n} \otimes P_{m}}}},{N_{t} = {2 \cdot M_{t}}}} & \left\lbrack {{Formula}\mspace{14mu} 14} \right\rbrack\end{matrix}$

A precoder for a case that a rank value is equal to the number of Txantennas (R=N_(t)) is configured using the P_(N) _(t) _(×N) _(t)obtained from Formula 14 and a column subset of the configured precodermay be usable as a precoder for a lower rank. If the precoder isconfigured in the above manner, the nested property can be met tosimplify the CQI calculation. In Formula 14, P_(N) _(t) _(×N) _(t)_(,n,m) indicates the precoder in case of R=N_(t). In this case, forexample, a subset configured with 0^(th) and 2^(nd) columns of P_(N)_(t) _(×N) _(t) _(,n,m) may be usable for a precoder for R=2, which canbe represented as P_(N) _(t) _(×N) _(t) _(n,m)(0,2). In this case, P_(M)_(t) _(×M) _(t) may be configured with a rotated DFT matrix or acodebook of another type.

Meanwhile, in order to raise a diversity gain in an open-loopenvironment, based on the precoder configured in the above manner, it isable to maximize the beam diversity gain by exchanging to use a precoderin accordance with a specific resource. For instance, in case of usingthe precoder according to the method of Formula 9, a method of applyinga precoder in accordance with a specific resource may be represented asFormula 15.

P _(N) _(t) _(×R,n,m)(k)=W _(k mod n) _(c)

P _(k mod m) _(c)   [Formula 15]

In Formula 15, k indicates a specific resource region. A precodingmatrix for a specific resource region k is determined by such a modulooperation as Formula 15. In this case, n_(c) and m_(c) may indicate asize or subset of a codebook for matrix W and a size or subset of acodebook for a matrix P, respectively.

Like Formula 15, if cycling is applied to each of the two matrixes,complexity may increase despite maximizing a diversity gain. Hence,long-term cycling may be set to be applied to a specific matrix andshort-term cycling may be set to be applied to the rest of the matrixes.

For instance, the matrix W may be configured to perform a modulooperation in accordance with a physical resource block (PRB) index andthe matrix P may be configured to perform a modulo operation inaccordance with a subframe index. Alternatively, the matrix W may beconfigured to perform a modulo operation in accordance with a subframeindex and the matrix P may be configured to perform a modulo operationin accordance with a physical resource block (PRB) index.

For another instance, the matrix W may be configured to perform a modulooperation in accordance with a physical resource block (PRB) index andthe matrix P may be configured to perform a modulo operation inaccordance with a subband index. Alternatively, the matrix W may beconfigured to perform a modulo operation in accordance with a subbandindex and the matrix P may be configured to perform a modulo operationin accordance with a physical resource block (PRB) index.

Moreover, a precoder cycling using a modulo operation is applied to oneof the two matrixes only and the other may be fixed to use.

Codebook Configuration for 8 Tx Antennas

In the 3GPP LTE Release-10 system having an extended antennaconfiguration (e.g., 8 Tx antennas), the feedback scheme used by thelegacy 3GPP LTE Release-8/9 may be applied in a manner of beingextended. For instance, it is able to feed back such channel stateinformation (CSI) as RI (Rank Indicator), PMI (Precoding Matrix Index),CQI (Channel Quality Information) and the like. In the followingdescription, a method of designing a dual precoder based feedbackcodebook usable for a system supportive of an extended antennaconfiguration is explained. In the dual precoder based feedbackcodebook, in order to indicate a precoder to be used for MIMOtransmission of a transmitting end, a receiving end may be able totransmit a precoding matrix index (PMI) to the transmitting end. Indoing so, a precoding matrix may be indicated by combination of 2different PMIs. In particular, the receiving end feeds back 2 differentPMIs (i.e., 1^(st) PMI and 2^(nd) PMI) to the transmitting end.Subsequently, the transmitting end determines the precoding matrixindicated by the combination of the 1^(st) and 2^(nd) PMIs and is thenable to apply the determined precoding matrix to the MIMO transmission.

In designing the dual precoder based feedback codebook, it may be ableto consider 8-Tx antenna MIMO transmission, single user-MIMO (SU-MIMO)and multiple user-MIMO (MU-MIMO) supports, compatibility with variousantenna configurations, codebook design references, codebook size andthe like.

As a codebook applicable to 8-Tx antenna MIMO transmission, it may beable to consider designing a feedback codebook. In particular, thisfeedback codebook supports SU-MIMO only in case of a rank higher than 2,is optimized for both SU-MIMO and MU-MIMO in case of a rank equal to orlower than 2, and is compatible with various antenna configurations.

In case of MU-MIMO, user equipments participating in MU-MIMO arepreferably separated in correlation domain. Hence, the codebook forMU-MIMO needs to be designed to correctly operate on a channel havinghigh correlation. Since DFT vectors provide good performance on achannel having high correlation, it may be able to consider having DFTvector contained in a codebook set of a rank up to a rank-2. In highscattering propagation environment (e.g., an indoor environment havingmany reflective waves, etc.) capable of producing many space channels,SU-MIMO operation may be preferred as the MIMO transmission scheme.Hence, it may be able to consider designing a codebook for a rank higherthan the rank-2 to have god performance in separating multiple layers.

In designing a precoder for MIMO transmission, it is preferable that oneprecoder structure has good performance for various antennaconfigurations (e.g., low-correlation, high-correlation, cross-pole,etc.). In arrangement of 8 Tx antennas, a cross-polarized array havingan antenna interval of 4λ may be formed in a low-correlation antennaconfiguration, a ULA having an antenna interval of 0.5λ may be formed ina high-correlation antenna configuration, or a cross-polarized arrayhaving an antenna interval of 0.5λ may be formed in a cross-polarizedantenna configuration. The DFT based codebook structure may be able toprovide good performance for the high-correlation antenna configuration.Meanwhile, block diagonal matrixes may be more suitable for thecross-polarized antenna configuration. Hence, in case that a diagonalmatrix is introduced into a codebook for 8 Tx antennas, it is able toconfigure a codebook that provides god performance for all antennaconfigurations.

As mentioned in the foregoing description, the codebook design referenceis to meet the unitary codebook, the CM property, the constrainedalphabet, the proper codebook size, the nested property and the like.This applies to the 3GPP LTE Release-8/9 codebook design. And, it may beable to consider applying such a codebook design reference to the 3GPPLTE Release-10 codebook design supportive of the extended antennaconfiguration.

Regarding the codebook size, it is necessary to increase the codebooksize to sufficiently support the advantage in using 8 Tx antennas. Inorder to obtain a sufficient precoding gain from 8 Tx antennas inenvironment with low correlation, a codebook in large size (e.g., acodebook in size over 4 bits for Rank 1 or Rank 2) may be required. Inorder to obtain a precoding gain in an environment with highcorrelation, a codebook in 4-bit size may be sufficient. Yet, in orderto achieve a multiplexing gain of MU-MIMO, it may be able to increase acodebook size for Rank 1 or Rank 2.

Based on the above description, general structures of a codebook for 8Tx antennas are explained as follows.

Codebook Structure (1)

In applying multi-granular feedback, a method of configuring a codebookfor 8 Tx antennas by combination of 2 base matrixes and a method ofconfiguring the combination of 2 base matrixes using an inner productare described as follows.

First of all, a method of using an inner product of 2 base matrixes maybe represented as Formula 16.

W={tilde over (W)} ₁ {tilde over (W)} ₂  [Formula 16]

In case that codebook for 8 Tx antennas is represented in form of aninner product, a 1^(st) base matrix may be represented as a diagonalmatrix shown in Formula 17 for a co-polarized antenna group.

$\begin{matrix}{{\overset{\sim}{W}}_{1} = {\begin{bmatrix}W_{1} & 0 \\0 & W_{1}\end{bmatrix}\left( {W_{1}\text{:}\mspace{14mu} {4N}} \right)}} & \left\lbrack {{Formula}\mspace{14mu} 17} \right\rbrack\end{matrix}$

Moreover, in case that a 2^(nd) base matrix is used to adjust a relativephase between polarizations, the 2^(nd) base matrix may be representedusing an identity matrix. For an upper rank of a codebook for 8 Txantennas, the 2^(nd) base matrix may be represented as Formula 18. InFormula 18, a relation between a coefficient ‘1’ of a 1^(st) row of the2^(nd) base matrix and a coefficient ‘a’ or ‘-a’ of a 2^(nd) row thereofmay be able to reflect the adjustment of a relative phase betweenorthogonal polarizations.

$\begin{matrix}{{\overset{\sim}{W}}_{2} = {\begin{bmatrix}I & I \\{aI} & {- {aI}}\end{bmatrix}\left( {I\text{:}\mspace{14mu} {NN}} \right)}} & \left\lbrack {{Formula}\mspace{14mu} 18} \right\rbrack\end{matrix}$

Hence, if the codebook for 8 Tx antennas is represented using the 1^(st)base matrix and the 2^(nd) base matrix, it can be represented as Formula19.

$\begin{matrix}{W = {{\begin{bmatrix}W_{1} & 0 \\0 & W_{1}\end{bmatrix}\begin{bmatrix}I & I \\{aI} & {- {aI}}\end{bmatrix}} = \begin{bmatrix}W_{1} & W_{1} \\{aW}_{1} & {- {aW}_{1}}\end{bmatrix}}} & \left\lbrack {{Formula}\mspace{14mu} 19} \right\rbrack\end{matrix}$

The codebook expressed using the inner product like Formula 19 can besimplified into Formula 20 using Kroneker product.

W=W ₂

W ₁(W ₁:4×N,W ₂:2×M)  [Formula 20]

In Formula 20, a precoding matrix included in a codebook W includes 4*2rows and N*M columns. Hence, it can be used as a codebook for 8 Txantennas and transmission of Rank ‘N*M’. For instance, in case ofconfiguring a codebook for 8 Tx antennas and transmission of Rank R, ifW₂ is configured with 2×M, a value N for W₁ becomes R/M. For instance,in case of configuring a codebook for 8 Tx antennas and transmission ofRank 4, if W₂ is configured with 2×2 (i.e., M=2) matrix (e.g., thematrix shown in Formula 13), W₁ may apply 4×2 (i.e., N=R/M=4/2=2) matrix(e.g., DFT matrix).

Codebook Structure (2)

Another method of configuring a codebook for 8 Tx antennas bycombination of 2 base matrixes is described as follows. Assuming thatthe 2 base matrixes are set to W1 and W2, respectively, a precodingmatrix W for configuring a codebook may be defined in form of W1*W2. ForRank 1 to Rank 8, W1 may be able to have such a form of a block diagonalmatrix as

$\begin{bmatrix}X & 0 \\0 & X\end{bmatrix}.$

For Rank 1 to Rank 4, X corresponding to a block of a block diagonalmatrix W1 may be configured with a matrix in size of 4×Nb. And, 16 4TxDFT beams can be defined for the X. In this case, beams indexes may begiven as 0, 1, 2, . . . , and 15, respectively. For each W1, theadjacent overlapping beams may be usable to reduce an edge effect infrequency-selective precoding. Hence, even if a codebook is configuredusing the same W1 for an identical or different W2, optimal performancecan be secured for several subbands.

For Rank 1 and Rank 2, X corresponding to a block diagonal matrix W1 maybe configured with a matrix in size of 4×4 (i.e., Nb=4). For each ofRank 1 and Rank 2, 8 W1 matrixes can be defined. And, one W1 may includebeams overlapping with the adjacent W1. In case that beam indexes aregiven as 0, 1, 2, . . . , and 15, respectively, for example, it is ableto configure 8 W1 matrixes, of which beams overlapping with the adjacentW1 matrix, such as {0, 1, 2, 3}, {2, 3, 4, 5}, {4, 5, 6, 7}, {6, 7, 8,9},{8, 9, 10, 11}, {10, 11, 12, 13}, {12, 13, 14, 15}, and {14, 15, 0,1}. For instance, a W1 codebook for Rank 1 or Rank 2 may be defined asFormula 21.

$\begin{matrix}{X^{(n)} = {\frac{1}{2} \times \begin{bmatrix}1 & 0 & 0 & 0 \\0 & ^{j\frac{\pi}{4}n} & 0 & 0 \\0 & 0 & ^{{j{(2)}}\frac{\pi}{4}n} & 0 \\0 & 0 & 0 & ^{{j{(3)}}\frac{\pi}{4}n}\end{bmatrix}{\quad{\begin{bmatrix}1 & 1 & 1 & 1 \\1 & ^{j\frac{\pi}{8}} & ^{{j{(2)}}\frac{\pi}{8}} & ^{{j{(3)}}\frac{\pi}{8}} \\1 & ^{{j{(2)}}\frac{\pi}{8}} & ^{{j{(2)}}{(2)}\frac{\pi}{8}} & ^{{j{(3)}}{(2)}\frac{\pi}{8}} \\1 & ^{{j{(3)}}\frac{\pi}{8}} & ^{{j{(2)}}{(3)}\frac{\pi}{8}} & ^{{j{(3)}}{(3)}\frac{\pi}{8}}\end{bmatrix},\mspace{79mu} {n = 0},1,2,\ldots \mspace{14mu},{{7\mspace{79mu} W_{1}^{(n)}} = \begin{bmatrix}X^{(n)} & 0 \\0 & X^{(n)}\end{bmatrix}},\mspace{79mu} {{CB}_{1} = \left\{ {W_{1}^{(0)},W_{1}^{(1)},W_{1}^{(2)},\ldots \mspace{14mu},W_{1}^{(7)}} \right\}}}}}} & \left\lbrack {{Formula}\mspace{14mu} 21} \right\rbrack\end{matrix}$

In Formula 21, X(n) corresponding to a block of a block diagonal matrixW1^((n)) is defined and a W1 codebook (CB₁) can be configured with 8different W1's.

Considering the selection and common-phase component of W2, theselection of 4 kinds of different matrixes is possible for Rank 1 and 4kinds of different QPSK co-phases are applicable for Rank 1. Hence,total 16 W2 matrixes can be defined. For instance, the W2 codebook (CB₂)for Rank 1 can be configured as Formula 22.

$\begin{matrix}{{{W_{2} \in {CB}_{2}} = \left\{ {{\frac{1}{\sqrt{2}}\begin{bmatrix}Y \\Y\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y \\{jY}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y \\{- Y}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y \\{- {jY}}\end{bmatrix}}} \right\}}\mspace{79mu} {Y \in \left\{ {\begin{bmatrix}1 \\0 \\0 \\0\end{bmatrix},\begin{bmatrix}0 \\1 \\0 \\0\end{bmatrix},\begin{bmatrix}0 \\0 \\1 \\0\end{bmatrix},\begin{bmatrix}0 \\0 \\0 \\1\end{bmatrix}} \right\}}} & \left\lbrack {{Formula}\mspace{14mu} 22} \right\rbrack\end{matrix}$

For Rank 2, the selection of 4 kinds of different matrixes is possibleand 2 kinds of different QPSK co-phases are applicable. Hence, total 8W2 matrixes can be defined. For instance, the W2 codebook (CB₂) for Rank2 can be configured as Formula 23.

$\begin{matrix}{{{W_{2} \in {CB}_{2}} = \left\{ {{\frac{1}{\sqrt{2}}\begin{bmatrix}Y & Y \\Y & {- Y}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y & Y \\{jY} & {- {jY}}\end{bmatrix}}} \right\}}{Y \in \left\{ {\begin{bmatrix}1 \\0 \\0 \\0\end{bmatrix},\begin{bmatrix}0 \\1 \\0 \\0\end{bmatrix},\begin{bmatrix}0 \\0 \\1 \\0\end{bmatrix},\begin{bmatrix}0 \\0 \\0 \\1\end{bmatrix}} \right\}}} & \left\lbrack {{Formula}\mspace{14mu} 23} \right\rbrack\end{matrix}$

Subsequently, for Rank 3 and Rank 4, X corresponding to a block diagonalmatrix W1 may be configured with a matrix in size of 4×8 (i.e., Nb=8).For each of Rank 3 and Rank 4, 4 W1 matrixes can be defined. And, one W1may include beams overlapping with the adjacent W1. In case that beamindexes are given as 0, 1, 2, . . . , and 15, respectively, for example,it is able to configure 4 W1 matrixes, of which beams overlapping withthe adjacent W1 matrix, such as {0, 1, 2, . . . , 7}, {4, 5, 6, . . . ,11}, {8, 9, 10, . . . , 15}, and {12, . . . , 15, 0, . . . , 3}. Forinstance, a W1 codebook for Rank 3 or Rank 4 may be defined as Formula24.

$\begin{matrix}{X^{(n)} = {\frac{1}{2} \times \begin{bmatrix}1 & 0 & 0 & 0 \\0 & (j)^{n} & 0 & 0 \\0 & 0 & \left( {- 1} \right)^{n} & 0 \\0 & 0 & 0 & \left( {- j} \right)^{n}\end{bmatrix}{\quad{\begin{bmatrix}1 & 1 & 1 & \ldots & 1 \\1 & ^{j\frac{\pi}{8}} & ^{{j{(2)}}\frac{\pi}{8}} & \ldots & ^{{j{(7)}}\frac{\pi}{8}} \\1 & ^{{j{(2)}}\frac{\pi}{8}} & ^{{j{(2)}}{(2)}\frac{\pi}{8}} & \ldots & ^{{j{(7)}}{(2)}\frac{\pi}{8}} \\1 & ^{{j{(3)}}\frac{\pi}{8}} & ^{{j{(2)}}{(3)}\frac{\pi}{8}} & \ldots & ^{{j{(7)}}{(3)}\frac{\pi}{8}}\end{bmatrix},\mspace{79mu} {n = 0},1,2,{{3\mspace{79mu} W_{1}^{(n)}} = \begin{bmatrix}X^{(n)} & 0 \\0 & X^{(n)}\end{bmatrix}},\mspace{79mu} {{CB}_{1} = \left\{ {W_{1}^{(0)},W_{1}^{(1)},W_{1}^{(2)},W_{1}^{(3)}} \right\}}}}}} & \left\lbrack {{Formula}\mspace{14mu} 24} \right\rbrack\end{matrix}$

In Formula 24, X(n) corresponding to a block of a block diagonal matrixW1^((n)) is defined and a W1 codebook (CB₁) can be configured with 4different W1's.

Considering the selection and common-phase component of W2, theselection of 8 kinds of different matrixes is possible for Rank 3 and 2kinds of different QPSK co-phases are applicable for Rank 3. Hence,total 16 W2 matrixes can be defined. For instance, the W2 codebook (CB₂)for Rank 3 can be configured as Formula 25.

$\begin{matrix}{{{W_{2} \in {CB}_{2}} = \left\{ {{\frac{1}{\sqrt{2}}\begin{bmatrix}Y_{1} & Y_{2} \\Y_{1} & {- Y_{2}}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y_{1} & Y_{2} \\{jY}_{1} & {- {jY}_{2}}\end{bmatrix}}} \right\}}{\left( {Y_{1},Y_{2}} \right) \in {\quad \begin{Bmatrix}\begin{matrix}{\left( {e_{1}\begin{bmatrix}e_{1} & e_{5}\end{bmatrix}} \right),\left( {e_{2}\begin{bmatrix}e_{2} & e_{6}\end{bmatrix}} \right),} \\{\left( {e_{3}\begin{bmatrix}e_{3} & e_{7}\end{bmatrix}} \right),\left( {e_{4}\begin{bmatrix}e_{4} & e_{8}\end{bmatrix}} \right),}\end{matrix} \\\begin{matrix}{\left( {e_{5}\begin{bmatrix}e_{1} & e_{5}\end{bmatrix}} \right),\left( {e_{6}\begin{bmatrix}e_{2} & e_{6}\end{bmatrix}} \right),} \\{\left( {e_{7}\begin{bmatrix}e_{3} & e_{7}\end{bmatrix}} \right),\left( {e_{8}\begin{bmatrix}e_{4} & e_{8}\end{bmatrix}} \right)}\end{matrix}\end{Bmatrix}}}} & \left\lbrack {{Formula}\mspace{14mu} 25} \right\rbrack\end{matrix}$

In Formula 24, e_(n) indicates 8×1 vector, n^(th) element has a value of1, and the rest of elements mean a selection vector having a value of 0.

For Rank 4, the selection of 4 kinds of different matrixes is possibleand 2 kinds of different QPSK co-phases are applicable. Hence, total 8W2 matrixes can be defined. For instance, the W2 codebook (CB₂) for Rank4 can be configured as Formula 26.

$\begin{matrix}{{{W_{2} \in {CB}_{2}} = \left\{ {{\frac{1}{\sqrt{2}}\begin{bmatrix}Y_{1} & Y_{2} \\Y_{1} & {- Y_{2}}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y_{1} & Y_{2} \\{jY}_{1} & {- {jY}_{2}}\end{bmatrix}}} \right\}}{Y \in \left\{ {\begin{bmatrix}e_{1} & e_{5}\end{bmatrix},\begin{bmatrix}e_{2} & e_{6}\end{bmatrix},\begin{bmatrix}e_{3} & e_{7}\end{bmatrix},\begin{bmatrix}e_{4} & e_{8}\end{bmatrix}} \right\}}} & \left\lbrack {{Formula}\mspace{14mu} 26} \right\rbrack\end{matrix}$

For Rank 5 to Rank 8, X corresponding to a block of a block diagonalmatrix W1 can be configured with DFT matrix in size of 4×4 and one W1matrix can be defined. W2 may be defined as a product of a matrix

$\begin{bmatrix}I & I \\I & {- I}\end{bmatrix}\quad$

and a row selection matrix in a fixed size of 8×r. For Rank 5, sinceselection of 4 kinds of different matrixes is possible, 4 W2 matrixescan be defined. For Rank 6, since selection of 4 kinds of differentmatrixes is possible, 4 W2 matrixes can be defined. For Rank 7, sinceselection of 1 kind of a matrix is possible, one W2 matrix can bedefined. For Rank 8, since selection of 1 kind of a matrix is possible,one W2 matrix can be defined. In this case, the matrix

$\begin{bmatrix}I & I \\I & {- I}\end{bmatrix}\quad$

is introduced to enable all polarized groups for each transmission layerto be identically used and good performance may be expected for atransmission of a high rank having a spatial channel having morescattering. In this case, the I means an identity matrix.

For instance, the W1 codebook or the W2 codebook for Rank 5 to Rank 8can be defined as Formula 27.

$\begin{matrix}{{{X = {\frac{1}{2} \times \begin{bmatrix}1 & 1 & 1 & 1 \\1 & j & {- 1} & {- j} \\1 & {- 1} & 1 & {- 1} \\1 & {- j} & {- 1} & j\end{bmatrix}}},{W_{1} = \begin{bmatrix}X & 0 \\0 & X\end{bmatrix}},{{CB}_{1} = \left\{ W_{1} \right\}}}{{W_{2} \in {CB}_{2}} = \left\{ {{\frac{1}{\sqrt{2}}\begin{bmatrix}I_{4} & I_{4} \\I_{4} & {- I_{4}}\end{bmatrix}}Y} \right\}}} & \left\lbrack {{Formula}\mspace{14mu} 27} \right\rbrack\end{matrix}$

In Formula 27, the W1 codebook for Rank 5 to Rank 8 is configured withone W1 matrix only. I₄ in the W2 codebook for Rank 5 to Rank 8 means anidentity matrix in size of 4×4. In Formula 27, a matrix Y can be definedas one of Formula 28 to Formula 31 for example.

The matrix Y for Rank 5 can be defined as Formula 28.

$\begin{matrix}{Y \in \begin{Bmatrix}{\begin{bmatrix}e_{1} & e_{2} & e_{3} & e_{4} & e_{5}\end{bmatrix},\begin{bmatrix}e_{2} & e_{3} & e_{4} & e_{5} & e_{6}\end{bmatrix},} \\{\begin{bmatrix}e_{3} & e_{4} & e_{5} & e_{6} & e_{7}\end{bmatrix},\begin{bmatrix}e_{4} & e_{5} & e_{6} & e_{7} & e_{8}\end{bmatrix},}\end{Bmatrix}} & \left\lbrack {{Formula}\mspace{14mu} 28} \right\rbrack\end{matrix}$

The matrix Y for Rank 6 can be defined as Formula 29.

$\begin{matrix}{Y \in {\begin{Bmatrix}{\begin{bmatrix}e_{1} & e_{2} & e_{3} & e_{4} & \begin{matrix}e_{5} & e_{6}\end{matrix}\end{bmatrix},\begin{bmatrix}e_{2} & e_{3} & e_{4} & e_{5} & \begin{matrix}e_{6} & e_{7}\end{matrix}\end{bmatrix},} \\{\begin{bmatrix}e_{3} & e_{4} & e_{5} & e_{6} & \begin{matrix}e_{7} & e_{8}\end{matrix}\end{bmatrix},\begin{bmatrix}e_{4} & e_{5} & e_{6} & e_{7} & \begin{matrix}e_{8} & e_{9}\end{matrix}\end{bmatrix},}\end{Bmatrix}\quad}} & \left\lbrack {{Formula}\mspace{14mu} 29} \right\rbrack\end{matrix}$

The matrix Y for Rank 7 can be defined as Formula 30.

Y=[e ₁ e ₂ e ₃ e ₄ e ₅ e ₆ e ₇]  [Formula 30]

The matrix Y for Rank 8 can be defined as Formula 31.

Y=I ₈  [Formula 31]

In Formula 31, the I₈ means 8×8 identity matrix.

As mentioned in the foregoing description, the numbers of W1's, whichcan be defined for each of Rank 1 to Rank 8, are added up to result in28 (=8+8+4+4+1+1+1+1).

Based on the above-mentioned description, explained is a method for atransmitting end to use partial antenna ports in case of transmittingCSI-RS (channel state information reference signal) as a technologybased 3D MIMO system in which a 2-dimensional active antenna system(2D-AAS) proposed in the present invention is installed.

Referring to FIG. 24, an antenna system that utilizes AAS is described.After LTE Rel-12, the antenna system utilizing the AAS shown in FIG. 24has been discussed. Since each antenna in the AAS corresponds to anactive antenna including an active circuit, an antenna pattern can bechanged depending on a situation. Thus, it is more efficient in reducinginterference or performing beamforming. Moreover, if the AAS isestablished in two dimensions (i.e., 2D-AAS), in aspect of the antennapattern, a main lobe of the antenna is adjusted more efficiently in 3dimensions. Thus, it is possible to actively change a transmitting beamdepending on a location of the receiving end. Accordingly, the 2D-AAScan be established as a system having multiple antennas in a manner ofarranging antennas vertically and horizontally as shown in FIG. 24.

Moreover, if the 2D-AAS is introduced, a transmitting end needs totransmit CSI-RS in order to inform a receiving end of a channel from thetransmitting end to the receiving end. In this regard, in the legacy LTEsystem (i.e., system before LTE/LTE-A release 11), CSI-RS is designed as2 ports, 4 ports, or 8 ports CSI-RS. In each n-ports CSI-RS, n resourceelements should be used in one RB. Thus, if the 2D-AAS has total 64antennas by arranging 8 antennas in a vertical direction and 8 antennasin a horizontal direction, 64 REs are used in one RB for CSI-RSaccording to a conventional scheme. In this case, it may cause a problemthat CSI-RS overhead increases depending on the number of antennas. Tosolve the above problem, discussion is ongoing with respect to a methodof estimating channels from remaining ports using partial CSI-RS ports.

Hereinafter, for convenience of explanation, it is assumed that channelsfrom 2D-AAS to a receiving end may be expressed as Kronecker product asshown in Formula 32.

$\begin{matrix}{H = {\begin{bmatrix}H_{T}^{(1)} \\H_{T}^{(2)} \\\vdots \\H_{T}^{(j)} \\\vdots \\H_{T}^{(N_{R})}\end{bmatrix} = \begin{bmatrix}{H_{V}^{(1)} \otimes H_{H}^{(1)}} \\{H_{V}^{(2)} \otimes H_{H}^{(2)}} \\\vdots \\{H_{V}^{(j)} \otimes H_{H}^{(j)}} \\\vdots \\{H_{V}^{(N_{R})} \otimes H_{H}^{(N_{R})}}\end{bmatrix}}} & \left\lbrack {{Formula}\mspace{14mu} 32} \right\rbrack\end{matrix}$

In Formula 32, H means entire channels from a transmitting end to areceiving end and H_(T) ^((j)) means a channel from the transmitting endto j^(th) receiving antenna. H_(V) ^((j)) and H_(H) ^((j)) mean achannel from a vertical direction of an antenna element (or port) toj^(th) antenna in the receiving end and a channel from a horizontaldirection of an antenna element (or port) to the j^(th) antenna in thereceiving end, respectively. Referring to FIG. 24, H_(V) ^((j)) means achannel from an antenna in A block to the j^(th) antenna in thereceiving end on the assumption that there are antennas in A block only.And, H_(H) ^((j)) means a channel from an antenna in B block to thej^(th) antenna in the receiving end on the assumption that there areantennas in B block only.

For convenience of explanation, the present invention is described onthe assumption that there is one random receiving antenna. However, allprocesses can be equally applied to a case that there are multipleantennas. That is, the present invention is described based on only achannel from a transmitting end to one random receiving antenna withoutindex (j) as shown in Formula 33. However, Formula 33 is only to explainthe present invention and the invention can be also applied to a case inwhich an actual channel is different from Formula 33.

H _(T) =H _(V)

H _(H)  [Formula 33]

According to the legacy wireless communication system, two CSI-RSs areconfigured in a manner of configuring one CSI-RS with N_(V) antennaports in the vertical direction as A block in FIG. 24 and one CSI-RSwith N_(H) antenna ports in the horizontal direction as B block in FIG.24. After measuring two received CSI-RSs, the receiving end may estimatea channel by performing the Kronecker product on two channel matrices asshown in Formula 32. Here, N_(V) is the number of antennas in thevertical direction and N_(H) is the number of antennas in the horizontaldirection. Such a scheme in the legacy wireless communication system hasan advantage of informing the receiving end of a channel from 64-portusing existing 2, 4 or 8-port CSI-RS only.

However, as it can be seen in FIG. 24, in case of using two 8-portCSI-RSs, one port is repeatedly transmitted since A block overlaps withB block in part. In other words, the receiving end receives a channelfrom the one port twice. In this case, a scheme of transmitting one8-port CSI-RS and one 7-port CSI-RS can be considered but it may cause aproblem of additionally creating the 7-port CSI-RS, which does not existin the current LTE.

Therefore, the present invention proposes a method of facilitatingchannel estimation by using a different antenna port for CSI-RS 1-port,which is repeatedly transmitted since it belongs to the verticaldirection of the antenna port and the horizontal direction of theantenna port at the same time as mentioned in the foregoing description.In particular, since the actual channel is not formed in the same manneras Kronecker product operation as shown in Formula 32, the presentinvention has an advantage of using the remaining CSI-RS 1-port for moreaccurate channel estimation.

According to the present invention, first of all, k^(th) row is selectedfrom an antenna domain by the transmitting end in the 2D-AAS for thehorizontal direction of the antennas. And, l^(th) column is selectedfrom the antenna domain for the vertical direction of the antennas. Bothof the k and l may be previously determined between the transmitting endand the receiving end. Alternatively, they may be informed through RRCsignaling or CSI-RS configuration.

Thereafter, the transmitting end configures two CSI-RSs. One of them isN_(V)-port CSI-RS (hereinafter referred to as V-CSI-RS) and the other isN_(H)-port CSI-RS (hereinafter referred to as H-CSI-RS). CSI-RS used inthe current LTE can be reused since the number of antennas in thevertical direction and the number of antennas in the horizontaldirection are assumed to be 1, 2, 4, or 8 in the current LTE system.However, in case of N_(V)=1, only one N_(H)-port H-CSI-RS is configured.And, in case of N_(H)=1, only one N_(V)-port V-CSI-RS is configured.Thus, it is preferred to apply the present invention to a case ofN_(V)>1 and N_(H)>1, except a case of N_(V)=1 or N_(H)=1.

According to the present invention, N_(H) antenna ports in the k^(th)row selected from the antenna domain may be transmitted throughN_(H)-port H-CSI-RS and N_(V)=1 antenna ports except k^(th) antenna portamong antenna ports in the l^(th) column selected from the antennadomain may be transmitted through N_(V)-port V-CSI-RS. In this case, oneport unused in N_(V)-port V-CSI-RS is used for one among antenna portsunselected from the antenna domain. Moreover, information on one antennaport with respect to the selected specific row (i.e., k^(th) row) may bepreviously determined between the transmitting end and the receivingend. Alternatively, the information may be informed through RRCsignaling or CSI-RS configuration.

FIG. 25 is a reference diagram to describe a method of transmittingCSI-RS according to an embodiment of the present invention. Referring toFIG. 25, the number of antennas in the vertical direction and the numberof antennas in the horizontal direction are 4, respectively. And, thetotal number of ports is 16 For example, after designating two 4-portCSI-RSs, a transmitter transmits antenna ports in a 2^(nd) row through4-port H-CSI-RS and antenna ports among antenna ports in a 2^(nd) columnexcept a 2^(nd) antenna port though 4-port V-CSI-RS.

In this case, one port unused in V-CSI-RS may transmit CSI-RS withrespect to a specific antenna port among the rest of antenna ports,which are not transmitted through H-CSI-RS and V-CSI-RS in an entiredomain antenna configuration of the antenna domain. FIG. 24 illustratesan example of using it for an antenna port corresponding to (4, 4)location. However, in some cases, CSI-RS with respect to a specificantenna port configured through preset standard/RRC signaling may betransmitted. For example, it may be configured to transmit CSI-RS withrespect to an antenna port having a lowest correlation with a selectedantenna port or CSI-RS with respect to an antenna port, which isphysically located farthest away from selected antennas.

Moreover, according to the present invention, CSI-RS 1-port, which issupposed to be transmitted repeatedly, is not transmitted. Instead,power boosting can be performed on CSI-RS having non-transmitted CSI-RSport as much as transmission power used for transmitting the CSI-RS1-port.

In particular, N_(H) antenna ports in the k^(th) row selected from theantenna domain are transmitted through N_(H)-port H-CSI-RS and N_(V)−1antenna ports except the k^(th) antenna port among antenna ports in thel^(th) column selected from the antenna domain are transmitted throughN_(V)-port V-CSI-RS. In this case, the power can be reduced since CDM(code division multiplexing) is performed on N_(V)−1 ports instead ofN_(V) ports in N_(V)-port V-CSI-RS transmission. Thus, in the case oftransmitting N_(V)-port V-CSI-RS, CDM is performed on N_(V)−1 ports andpower boosting is performed as much as when CDM is performed on N_(V)ports.

More particularly, it is currently configured that CDM is performed onCSI-RS in a manner of making a bundle of two REs. Thus, if CDM isperformed on N_(V)−1 ports using N_(V)-port V-CSI-RS, half of power maybe used in some two REs. Therefore, in this case, two REs having thereduced power may be transmitted using double power or all the REs usedin V-CSI-RS may be power-boosted as much as

$\sqrt{\frac{N_{V}}{N_{V} - 1}}.$

Moreover, since UE knows which CSI-RS transmits one less port, the UE isable to estimate how much power is used for CSI-RS power boosting.

Furthermore, according to the present invention, when an actual channelis significantly different from the Kronecker product form of thechannel shown in Formula 32, the transmitter may use two CSI-RSscorresponding to N_(A)-port H-CSI-RS and N_(B)-port V-CSI-RS. In thiscase, N_(A) and N_(B) needs to satisfy conditions of N_(A)≧N_(H) andN_(B)≧N_(V), respectively. These conditions means that the number ofports defined for H-CSI-RS port is equal to or greater than the numberof antennas in the horizontal direction and the number of ports definedfor V-CSI-RS port is equal to or greater than the number of antennas inthe vertical direction.

Thus, N_(H) antenna ports in the k^(th) row selected from the antennadomain are transmitted through N_(A)-port H-CSI-RS and N_(V)−1 antennaports except the k^(th) antenna port among antenna ports in the l^(th)column selected from the antenna domain are transmitted throughN_(B)-port V-CSI-RS. And, N_(A)−N_(H) ports unused in H-CSI-RS andN_(B)−N_(V)+1 ports unused in V-CSI-RS are used for N_(A)−N_(H)N_(B)−N_(V)+1 ports among ports unused as CSI-RS in the antenna domain.

Information on N_(A)−N_(H)+N_(B)−N_(V)+1 antenna ports may be previouslydetermined between the transmitting end and the receiving end.Alternatively, the information may be informed through RRC signaling orCSI-RS configuration.

Moreover, the present invention can be applied to a case that N_(H)−1antenna ports except l^(th) antenna port among antenna ports in thek^(th) row selected from the antenna domain are transmitted throughN_(H)-port H-CSI-RS and N_(V) antenna ports in the l^(th) columnselected from the antenna domain are transmitted through N_(V)-portV-CSI-RS. In this case, one port unused in N_(H)-port H-CSI-RS maytransmit CSI-RS with respect to a specific antenna port among the restof antenna ports, which are not transmitted through H-CSI-RS andV-CSI-RS in an entire domain antenna configuration of the antennadomain. Further, information on one antenna port with respect to theselected specific row (i.e., k^(th) row) may be previously determinedbetween the transmitting end and the receiving end. Alternatively, theinformation may be informed through RRC signaling or CSI-RSconfiguration.

The power boosting may be performed when N_(H)−1 antenna ports exceptlth antenna port among antenna ports in the k^(th) row selected from theantenna domain are transmitted through N_(H)-port H-CSI-RS and N_(V)antenna ports in the l^(th) column selected from the antenna domain aretransmitted through N_(V)-port V-CSI-RS. In particular, the power can bereduced since CDM is performed on N_(H)−1 ports instead of N_(H) portsin N_(H)-port H-CSI-RS transmission. In other words, in the case oftransmitting N_(H)-port H-CSI-RS, CDM is performed on N_(H)−1 ports andpower boosting is performed as much as when CDM is performed on N_(H)ports.

More particularly, it is currently configured that CDM is performed onCSI-RS in a manner of making a bundle of two REs. Thus, if CDM isperformed on N_(H)−1 ports using N_(H)-port H-CSI-RS, half of power maybe used in some two REs. Therefore, in this case, two REs having thereduced power may be transmitted using double power or all the REs usedin H-CSI-RS may be power-boosted as much as

$\sqrt{\frac{N_{H}}{N_{H} - 1}}.$

As mentioned in the foregoing description, since UE knows which CSI-RStransmits one less port, the UE is able to estimate how much power isused for CSI-RS power boosting.

In addition, according to the present invention, when an actual channelis significantly different from the Kronecker product form of thechannel shown in Formula 32, the transmitter may use two CSI-RSscorresponding to N_(A)-port H-CSI-RS and N_(B)-port V-CSI-RS. RS. Inthis case, N_(A) and N_(B) needs to satisfy conditions of N_(A)≧N_(H)and N_(B)≧N_(V), respectively.

Therefore, N_(H)−1 antenna ports except lth antenna port among antennaports in the k^(th) row selected from the antenna domain are transmittedthrough N_(A)-port H-CSI-RS and N_(V) antenna ports in the l^(th) columnselected from the antenna domain are transmitted through N_(B)-portV-CSI-RS. And, N_(A)−N_(H)+1 ports unused in H-CSI-RS and N_(B)−N_(V)ports unused in V-CSI-RS are used for N_(A)−N_(H)+N_(B)−N_(V)+1 portsamong ports unused as CSI-RS in the antenna domain.

Information on N_(A)−N_(H)+N_(B)−N_(V)+1 antenna ports may be previouslydetermined between the transmitting end and the receiving end.Alternatively, the information may be informed through RRC signaling orCSI-RS configuration.

Although the present invention has been described focusing on CSI-RS, itwill be described hereinafter centering on CRS. In the current LTEwireless communication system, not only the CSI-RS but also the CRS aredefined.

That is, to reduce RS overhead, it is proposed that a base station and aUE (user equipment) combine channels by utilizing both of the CSI-RS andthe CRS for CSI measurement at the UE.

After measuring (or estimating) channels using CSI-RS ports and CRSports, the base station and the UE estimate entire channels by combiningchannels obtained from the CSI-RS ports and channels obtained from theCRS ports. Such a method of combining channels may be previously definedbetween the base station and the UE or it may be semi-statically definedthrough RRC signaling. As an example of the method of combiningchannels, a final channel can be created through the Kronecker product,which is shown in Formula 33, of the channels obtained from the CSI-RSports and the channels obtained from the CRS ports.

As an example of the method of combining channels using both of theCSI-RS and the CRS, which is proposed in the present invention, channelsin the vertical direction can be transmitted using the CSI-RS ports andchannels in the horizontal direction can be transmitted using the CRSports. In addition, the UE may estimate the entire channels by combiningthe channels obtained from the CSI-RS ports and the channels obtainedfrom the CRS-ports (for example, the UE may estimate the entire channelsthrough the Kronecker product of the CSI-RS based vertical channels andthe CRS based horizontal channels). Thereafter, the UE may calculate RI,PMI, and CQI based on the combined channels and then feed back thecalculated RI, PMI, and CQI to the base station.

FIG. 26 illustrates an embodiment of channel estimation using CSI-RS andCRS according to the present invention. Referring to FIG. 26, a basestation configures one 8 port CSI-RS for a vertical block (hereinafter,referred to as V block) and one 4 port CRS for a horizontal block(hereinafter, referred to as H block) for a UE. The UE estimates channelH_(V) for the V block from the configured 8 port CSI-RS and channelH_(H) for the H block from the configured 4 port CRS. Subsequently, theUE estimates entire channels H_(V)

H_(H) (or H_(H)

H_(V) through the Kronecker product of the two channels. Thereafter, theUE may calculate RI, PMI, and CQI based on the entire combined channelsand then feed back the calculated RI, PMI, and CQI to the base station.For convenience of description, FIG. 26 illustrates a case in whichantenna ports in the V block and antenna ports in the H block are used.However, it is apparent that the present invention can be applied to acase in which antenna ports in any vertical block instead of the V blockand antenna ports in any horizontal block instead of the H block areused.

Moreover, the present invention can also be applied to a case in whichhorizontal channels are transmitted using CSI-RS ports and verticalchannels are transmitted using CRS ports. The UE may estimate the entirechannels by combining the horizontal channels obtained from the CSI-RSports and the vertical channels obtained from the CRS-ports (forexample, the UE may estimate the entire channels through the Kroneckerproduct). Thereafter, the UE may calculate RI, PMI, and CQI based on theentire combined channels and then feed back the calculated RI, PMI, andCQI to the base station.

FIG. 27 illustrates another embodiment of channel estimation usingCSI-RS and CRS according to the present invention. Referring to FIG. 27,a base station configures one 8 port CSI-RS for a horizontal block(hereinafter, referred to as H block) and one 4 port CRS for a verticalblock (hereinafter, referred to as V block) for a UE. The UE estimateschannel H_(H) for the H block from the configured 8 port CSI-RS andchannel H_(V) for the V block from the configured 4 port CRS.Subsequently, the UE estimates entire channels H_(V)

H_(H) (or H_(H)

H_(V)) through the Kronecker product of the two channels. Thereafter,the UE may calculate RI, PMI, and CQI based on the entire combinedchannels and then feed back the calculated RI, PMI, and CQI to the basestation. For convenience of description, FIG. 27 illustrates a case inwhich antenna ports in the V block and antenna ports in the H block areused. However, it is apparent that the present invention can be appliedto a case in which antenna ports in any vertical block instead of the Vblock and antenna ports in any horizontal block instead of the H blockare used.

However, the channel estimation methods of using 8 port CSI-RS and 4port CRS for different channel blocks with different directions, whichare described with reference to FIGS. 26 and 27, cannot be applied insome cases. For instance, if the number of both of vertical andhorizontal antenna ports is greater than four, the aforementionedchannel estimation methods cannot be applied since the number of maximumCRS ports is defined as four.

FIG. 28 is a reference diagram illustrating a case in which the numberof both of vertical and horizontal antenna ports is greater than four.

Another embodiment of the present invention is described with referenceto FIG. 28. The present invention propose that CRS is used for specificantenna ports among all antenna ports included in one horizontal antennablock, and CSI-RS is used for remaining antenna ports except thespecific antenna ports in the horizontal antenna block and antenna portsin one vertical antenna block. On the contrary, CRS may be used forspecific antenna ports among all antenna ports included in one verticalantenna block, and CSI-RS may be used for remaining antenna ports exceptthe specific antenna ports in the vertical antenna block and antennaport in one horizontal antenna block. Similarly, in this case, a UE mayestimate entire channels after combining channels obtained using the CRSand the CSI-RS. Thereafter, the UE may calculate RI, PMI, and CQI basedon the estimated channels and then feed back the calculated RI, PMI, andCQI to a base station.

Referring to FIG. 28, the base station configures one 4 port CRS for ahorizontal block (hereinafter referred to as H block), one 4 port CSI-RSfor H2 block, and one 8 port CSI-RS for a vertical block (hereinafterreferred to as V block) for the UE. The UE estimates channel H_(H) for Hand H2 blocks from the configured 4 port CRS and 4 port CSI-RS andchannel H_(V) for the V block from the configured 8 port CSI-RS.Subsequently, the UE estimates entire channels H_(V)

H_(H) (or H_(H)

H_(V)) through the Kronecker product of the two channels. Thereafter,the UE may calculate the RI, PMI, and CQI based on the estimatedchannels and then feed back the calculated RI, PMI, and CQI to the basestation. For convenience of description, FIG. 28 illustrates a case inwhich antenna ports in the V block, antenna ports in the H block, andantenna ports in the H2 block are used. However, it is apparent that thepresent invention can be applied to a case in which antenna ports in anyvertical block instead of the V block, antenna ports in any horizontalblock instead of the H block, and antenna ports in any horizontal blockinstead of H2 blocks are used.

Further, in the embodiments of the present invention described withreference to FIGS. 26 to 28, one antenna port overlaps each of thevertical antenna ports and the horizontal antenna ports.

For the antenna port that overlaps each of the vertical antenna portsand the horizontal antenna ports, a reference signal may be configuredso that the vertical direction and the horizontal direction overlap eachother. However, one port among the vertical or horizontal antenna portsmay not be used. Alternatively, the antenna port that overlaps each ofthe vertical antenna ports and the horizontal antenna ports may be usedfor another antenna port.

For instance, the V block and H block use one port in common as shown inFIG. 28. That is, one port of 4 port CRS for the V block overlaps withone port of 8 port CSI-RS for the H block. Thus, the base station maynot transmit one of the two overlapping ports by setting its power to‘0’. Alternatively, the base station may transmit a reference signal foran antenna port except the antenna ports included in the V, H, and H2blocks using the one of the two overlapping ports. Moreover, such aconfiguration may be previously defined between the base station and theUE or it may be semi-statically changed through RRC signaling.

Furthermore, whether the aforementioned embodiments of the presentinvention are applied may be indicated through RRC signaling (in asemi-static manner).

Particularly, a description will be given of a method of indicatingwhether the embodiments of the present invention are applied through RRCsignaling. According to the CSI-RS configuration defined in the currentLTE standard, a CRS port quasi co-located with CSI-RS needs to bedesignated. That is, according to the 3GPP TS 36.331, the standardspecification for LTE Release 11, non-zero power CSI-RS configuration isdefined as shown in Table 7.

TABLE 7 -- ASN1START CSI-RS-ConfigNZP-r11 ::= SEQUENCE {csi-RS-ConfigNZPId-r11 CSI-RS-ConfigNZPId-r11, antennaPortsCount-r11ENUMERATED {an1, an2, an4, an3}, resourceConfig-r11 INTEGER {0..31},subframeConfig-r11 INTEGER {0..154}, scramblingIdentity-r11 INTEGER{0..503}, qcl-CRS-Info-r11 SEQUENCE {   qcl-ScramblingIdentity-r11  INTEGER (0..503),   crs-PortsCount-r11   ENUMERATED (n1, n2, n4,spare1),   mbsfn-SubframeConfigList-r11   CHOICE {       release     NULL,       setup      SEQUENCE {        subframeConfigList        NESFN-SubframeConfigList       }   } OPTIONAL  -- Need ON }OPTIONAL  -- Need OR ... } -- ASN1STOP

In Table 7, qcl-CRS-Info-r11 indicates information on CRS quasico-located with CSI-RS designated in the CSI-RS configuration. Thus,when the embodiments of the present invention are applied based on theassumption that entire channels are estimated (e.g., through theKronecker product) by combining channels measured through CSI-RS andCRS, a base station needs to inform a UE of CSI-RS and CRS which arebound to each other. Particularly, the base station can indicate theCSI-RS and CRS bound to each other through the following methods A to C.

-   -   Method A: Method of estimating channels combined by CSI-RS        designated in CSI-RS configuration and CRS quasi co-located with        the CSI-RS.    -   Method B: When channel are estimated by combining channels of        CSI-RS and CRS, the CRS is designated as CRS of a serving cell.    -   Method C: Both of the methods A and B are promised in advance        and their application is semi-statically indicated through RRC        signaling.

FIG. 29 is a diagram for configurations of a base station device and auser equipment device according to the present invention.

Referring to FIG. 29, a base station device 2910 according to thepresent invention may include a receiving module 2911, a transmittingmodule 2912, a processor 2913, a memory 2914 and a plurality of antennas2915. In this case, a plurality of the antennas 2915 may mean a basestation device that supports MIMO transmission and reception. Thereceiving module 2911 may receive various signals, data, information andthe like in uplink from a user equipment. The transmitting module 2912may transmit various signals, data, information and the like in downlinkto the user equipment. Moreover, the processor 2913 may be configured tocontrol overall operations of the base station device 2910.

The base station device 2910 according to one embodiment of the presentinvention may be configured to transmit a DL signal. And, the memory2914 of the base station device 2910 may store codebook includingprecoding matrixes. The processor 2913 of the base station device 2910may be configured to receive 1st PMI (precoding matrix indicator) and2nd PMI from the user equipment through the receiving module 2911. Theprocessor 2913 may be configured to determine a 1st matrix (W1) from 1stcodebook including the precoding matrixes indicated by the 1st PMI anddetermine a 2nd matrix (W2) from 2nd codebook including the precodingmatrixes indicated by the 2nd PMI. The processor 2913 may be configuredto determine a precoding matrix (W) based on the 1st matrix (W1) and the2nd matrix (W2). The processor 2913 may be configured to performprecoding on at least one layer, to which the DL signal is mapped, usingthe determined precoding matrix (W). The processor 2913 may beconfigured to transmit the precoded signal to the user equipment throughthe transmitting module 2912. In this case, each of the precodingmatrixes included in the 1st codebook includes a block diagonal matrix.And, one block may have a form multiplied by a prescribed phase value,which is different from a different block.

The processor 2913 of the base station device 2910 performs a functionof processing information received by the base station device 2910,information to be externally transmitted and the like. The memory 2914may store the processed information and the like for prescribed durationand be substituted with such a component as a buffer (not shown in thedrawing) or the like.

Referring to FIG. 29, a user equipment device 2920 according to thepresent invention may include a receiving module 2921, a transmittingmodule 2922, a processor 2923, a memory 2924 and a plurality of antennas2925. In this case, a plurality of the antennas 2925 may mean a userequipment device that supports MIMO transmission and reception. Thereceiving module 2921 may receive various signals, data, information andthe like in downlink from a base station. The transmitting module 2922may transmit various signals, data, information and the like in uplinkto the base station. Moreover, the processor 2923 may be configured tocontrol overall operations of the user equipment device 2920.

The user equipment device 2920 according to one embodiment of thepresent invention may be configured to receive and process a DL signal.And, the memory 2924 of the user equipment device 2920 may storecodebook including precoding matrixes. The processor 2923 of the userequipment device 2920 may be configured to transmit 1st PMI (precodingmatrix indicator) and 2nd PMI to the base station through thetransmitting module 2922. The processor 2923 may be configured toreceive the DL signal through the receiving module 2921. In this case,the DL signal received by the user equipment corresponds to the DLsignal precoded by the base station using the precoding matrix (W). Inparticular, the precoding may be performed by the base station on atleast one layer to which the DL signal is mapped. In this case, theprecoding matrix (W) may be determined based on the 1st matrix (W1)determined from the 1st codebook including the precoding matrixesindicated by the 1st PMI and the 2nd matrix (W2) determined from the 2ndcodebook including the precoding matrixes indicated by the 2nd PMI. Theprocessor 2913 may be configured to determine a precoding matrix (W)based on the 1st matrix (W1) and the 2nd matrix (W2). The processor 2913may be configured to process the received DL signal using the determinedprecoding matrix (W). In this case, each of the precoding matrixesincluded in the 1st codebook includes a block diagonal matrix. And, oneblock may have a form multiplied by a prescribed phase value, which isdifferent from a different block.

The processor 2923 of the user equipment device 2920 performs a functionof processing information received by the user equipment device 2920,information to be externally transmitted and the like. The memory 2924may store the processed information and the like for prescribed durationand be substituted with such a component as a buffer (not shown in thedrawing) or the like.

The detailed configurations of the base station device and the userequipment device mentioned in the above description may be implementedin a manner that the matters of various embodiments of the presentinvention mentioned in the foregoing description are independentlyapplied or that at least two embodiments of the present invention aresimultaneously applied. And, redundant contents may be omitted forclarity.

In the description with reference to FIG. 29, the description of thebase station device 2910 may be identically applicable to a relay deviceas a downlink transmission entity or an uplink reception entity. And,the description of the user equipment device 2920 may be identicallyapplicable to a relay device as a downlink reception entity or an uplinktransmission entity.

The embodiments of the present invention may be implemented usingvarious means. For instance, the embodiments of the present inventionmay be implemented using hardware, firmware, software and/or anycombinations thereof.

In case of the implementation by hardware, a method according to eachembodiment of the present invention may be implemented by at least oneselected from the group consisting of ASICs (application specificintegrated circuits), DSPs (digital signal processors), DSPDs (digitalsignal processing devices), PLDs (programmable logic devices), FPGAs(field programmable gate arrays), processor, controller,microcontroller, microprocessor and the like.

In case of the implementation by firmware or software, a methodaccording to each embodiment of the present invention can be implementedby modules, procedures, and/or functions for performing theabove-explained functions or operations. Software code may be stored ina memory unit and may be then drivable by a processor. The memory unitmay be provided within or outside the processor to exchange data withthe processor through the various means known to the public.

As mentioned in the foregoing description, the detailed descriptions forthe preferred embodiments of the present invention are provided to beimplemented by those skilled in the art. While the present invention hasbeen described and illustrated herein with reference to the preferredembodiments thereof, it will be apparent to those skilled in the artthat various modifications and variations can be made therein withoutdeparting from the spirit and scope of the invention. For instance, therespective configurations disclosed in the aforesaid embodiments of thepresent invention can be used by those skilled in the art in a manner ofbeing combined with one another. Therefore, the present invention isnon-limited by the embodiments disclosed herein but intends to give abroadest scope matching the principles and new features disclosedherein.

The present invention may be embodied in other specific forms withoutdeparting from the spirit and essential characteristics of theinvention. Thus, the above embodiments should be considered in allrespects as exemplary and not restrictive. The scope of the presentinvention should be determined by reasonable interpretation of theappended claims and the present invention covers the modifications andvariations of this invention that come within the scope of the appendedclaims and their equivalents. The present invention is non-limited bythe embodiments disclosed herein but intends to give a broadest scopematching the principles and new features disclosed herein. And, it isapparently understandable that an embodiment is configured by combiningclaims failing to have relation of explicit citation in the appendedclaims together or can be included as new claims by amendment afterfiling an application.

INDUSTRIAL APPLICABILITY

Although a method and apparatus for transmitting a reference signal in awireless communication system supporting multiple antennas are mainlydescribed with reference to the examples of applying to 3GPP LTE system,as mentioned in the foregoing description, the present invention isapplicable to various kinds of wireless communication systems as well asto the 3GPP LTE system.

What is claimed is:
 1. A method of estimating a channel by a userequipment in a wireless communication system supporting multipleantennas, the method comprising: receiving a CSI-RS (channel stateinformation-reference signal) for a plurality of first domain antennasand a CRS (cell-specific reference signal) for a plurality of seconddomain antennas; and estimating entire channels based on a first channelfor the plurality of the first domain antennas estimated from the CSI-RSand a second channel for the plurality of the second domain antennasestimated from the CRS.
 2. The method of claim 1, wherein the entirechannels are determined by Kronecker product of the first channel andthe second channel.
 3. The method of claim 1, wherein, when theplurality of the first domain antennas are vertical domain antennas, theplurality of the second domain antennas are horizontal domain antennasand wherein, when the plurality of the first domain antennas are thehorizontal domain antennas, the plurality of the second domain antennasare the vertical domain antennas.
 4. The method of claim 1, wherein theCSI-RS and the CRS are configured through RRC (radio resource control)signaling.
 5. The method of claim 1, wherein the CRS comprises a CRSquasi co-located with the CSI-RS.
 6. The method of claim 1, wherein theCRS comprises a CRS of a serving cell.
 7. The method of claim 1, furthercomprising feeding back channel state information on the entire channelsto a base station.
 8. A method of estimating a channel by a userequipment in a wireless communication system supporting multipleantennas, the method comprising: receiving a CSI-RS (channel stateinformation-reference signal) for a plurality of vertical antennas andfirst horizontal antennas and a CRS (cell-specific reference signal) forsecond horizontal antennas; and estimating entire channels based on avertical antenna channel estimated from the CSI-RS and a horizontalantenna channel estimated from the CSI-RS and the CRS.
 9. The method ofclaim 8, wherein the entire channels are determined by Kronecker productof the vertical antenna channel and the horizontal antenna channel. 10.A user equipment for performing channel estimation in a wirelesscommunication system supporting multiple antennas, comprising: a radiofrequency unit; and a processor, wherein the processor is configured toreceive a CSI-RS (channel state information-reference signal) for aplurality of first domain antennas and a CRS (cell-specific referencesignal) for a plurality of second domain antennas and estimate entirechannels based on a first channel for the plurality of the first domainantennas estimated from the CSI-RS and a second channel for theplurality of the second domain antennas estimated from the CRS.